MC: Restructure MCFragment as a fixed part and a variable tail

Refactor the fragment representation of `push rax; jmp foo; nop; jmp foo`,
previously encoded as
`MCDataFragment(nop); MCRelaxableFragment(jmp foo); MCDataFragment(nop); MCRelaxableFragment(jmp foo)`,

to

```
MCFragment(fixed: push rax, variable: jmp foo)
MCFragment(fixed: nop, variable: jmp foo)
```

Changes:

* Eliminate MCEncodedFragment, moving content and fixup storage to MCFragment.
* The new MCFragment contains a fixed-size content (similar to previous
  MCDataFragment) and an optional variable-size tail.
* The variable-size tail supports FT_Relaxable, FT_LEB, FT_Dwarf, and
  FT_DwarfFrame, with plans to extend to other fragment types.
  dyn_cast/isa should be avoided for the converted fragment subclasses.
* In `setVarFixups`, source fixup offsets are relative to the variable part's start.
  Stored fixup (in `FixupStorage`) offsets are relative to the fixed part's start.
  A lot of code does `getFragmentOffset(Frag) + Fixup.getOffset()`,
  expecting the fixup offset to be relative to the fixed part's start.
* HexagonAsmBackend::fixupNeedsRelaxationAdvanced needs to know the
  associated instruction for a fixup. We have to add a `const MCFragment &` parameter.
* In MCObjectStreamer, extend `absoluteSymbolDiff` to apply to
  FT_Relaxable as otherwise there would be many more FT_DwarfFrame
  fragments in -g compilations.

https://llvm-compile-time-tracker.com/compare.php?from=267b136359d8448c73432b4f3ceeefbf4c35e00b&to=ef026be147babfd2b8f4ed4d875deadcbbbb2bbb&stat=instructions:u

```
stage2-O0-g instructins:u geomeon (-0.12%)
stage1-ReleaseLTO-g (link only) max-rss geomean (-0.39%)
```

```
% /t/clang-old -g -c sqlite3.i -w -mllvm -debug-only=mc-dump &| awk '/^[0-9]+/{s[$2]++;tot++} END{print "Total",tot; n=asorti(s, si); for(i=1;i<=n;i++) print si[i],s[si[i]]}'
Total 59675
Align 2215
Data 29700
Dwarf 12044
DwarfCallFrame 4216
Fill 92
LEB 12
Relaxable 11396
% /t/clang-new -g -c sqlite3.i -w -mllvm -debug-only=mc-dump &| awk '/^[0-9]+/{s[$2]++;tot++} END{print "Total",tot; n=asorti(s, si); for(i=1;i<=n;i++) print si[i],s[si[i]]}'
Total 32287
Align 2215
Data 2312
Dwarf 12044
DwarfCallFrame 4216
Fill 92
LEB 12
Relaxable 11396
```

Pull Request: llvm#148544

Author: MaskRay

llvm/include/llvm/MC/MCAsmBackend.h

@@ -19,11 +19,8 @@
 namespace llvm {
 
 class MCAlignFragment;
-class MCDwarfCallFrameFragment;
-class MCDwarfLineAddrFragment;
 class MCFragment;
 class MCLEBFragment;
-class MCRelaxableFragment;
 class MCSymbol;
 class MCAssembler;
 class MCContext;
@@ -157,8 +154,9 @@ class LLVM_ABI MCAsmBackend {
 
   /// Target specific predicate for whether a given fixup requires the
   /// associated instruction to be relaxed.
-  virtual bool fixupNeedsRelaxationAdvanced(const MCFixup &, const MCValue &,
-                                            uint64_t, bool Resolved) const;
+  virtual bool fixupNeedsRelaxationAdvanced(const MCFragment &, const MCFixup &,
+                                            const MCValue &, uint64_t,
+                                            bool Resolved) const;
 
   /// Simple predicate for targets where !Resolved implies requiring relaxation
   virtual bool fixupNeedsRelaxation(const MCFixup &Fixup,
@@ -179,18 +177,16 @@ class LLVM_ABI MCAsmBackend {
   }
 
   // Defined by linker relaxation targets.
-  virtual bool relaxDwarfLineAddr(MCDwarfLineAddrFragment &DF,
-                                  bool &WasRelaxed) const {
+  virtual bool relaxDwarfLineAddr(MCFragment &, bool &WasRelaxed) const {
     return false;
   }
-  virtual bool relaxDwarfCFA(MCDwarfCallFrameFragment &DF,
-                             bool &WasRelaxed) const {
+  virtual bool relaxDwarfCFA(MCFragment &, bool &WasRelaxed) const {
     return false;
   }
 
   // Defined by linker relaxation targets to possibly emit LEB128 relocations
   // and set Value at the relocated location.
-  virtual std::pair<bool, bool> relaxLEB128(MCLEBFragment &LF,
+  virtual std::pair<bool, bool> relaxLEB128(MCFragment &,
                                             int64_t &Value) const {
     return std::make_pair(false, false);
   }
@@ -228,7 +224,7 @@ class LLVM_ABI MCAsmBackend {
 
   bool isDarwinCanonicalPersonality(const MCSymbol *Sym) const;
 
-  // Return STI for fragments of type MCRelaxableFragment and MCDataFragment
+  // Return STI for fragments of type MCRelaxableFragment and MCFragment
   // with hasInstructions() == true.
   static const MCSubtargetInfo *getSubtargetInfo(const MCFragment &F);
 };

llvm/include/llvm/MC/MCAssembler.h

@@ -34,13 +34,10 @@ namespace llvm {
 class MCBoundaryAlignFragment;
 class MCCVDefRangeFragment;
 class MCCVInlineLineTableFragment;
-class MCDwarfCallFrameFragment;
-class MCDwarfLineAddrFragment;
-class MCEncodedFragment;
+class MCFragment;
 class MCFixup;
 class MCLEBFragment;
 class MCPseudoProbeAddrFragment;
-class MCRelaxableFragment;
 class MCSymbolRefExpr;
 class raw_ostream;
 class MCAsmBackend;
@@ -102,7 +99,7 @@ class MCAssembler {
 
   /// Check whether a fixup can be satisfied, or whether it needs to be relaxed
   /// (increased in size, in order to hold its value correctly).
-  bool fixupNeedsRelaxation(const MCRelaxableFragment &, const MCFixup &) const;
+  bool fixupNeedsRelaxation(const MCFragment &, const MCFixup &) const;
 
   void layoutSection(MCSection &Sec);
   /// Perform one layout iteration and return the index of the first stable
@@ -111,11 +108,11 @@ class MCAssembler {
 
   /// Perform relaxation on a single fragment.
   bool relaxFragment(MCFragment &F);
-  bool relaxInstruction(MCRelaxableFragment &IF);
-  bool relaxLEB(MCLEBFragment &IF);
+  bool relaxInstruction(MCFragment &F);
+  bool relaxLEB(MCFragment &F);
   bool relaxBoundaryAlign(MCBoundaryAlignFragment &BF);
-  bool relaxDwarfLineAddr(MCDwarfLineAddrFragment &DF);
-  bool relaxDwarfCallFrameFragment(MCDwarfCallFrameFragment &DF);
+  bool relaxDwarfLineAddr(MCFragment &F);
+  bool relaxDwarfCallFrameFragment(MCFragment &F);
   bool relaxCVInlineLineTable(MCCVInlineLineTableFragment &DF);
   bool relaxCVDefRange(MCCVDefRangeFragment &DF);
   bool relaxFill(MCFillFragment &F);

llvm/include/llvm/MC/MCCodeView.h

@@ -26,7 +26,6 @@ namespace llvm {
 class MCAssembler;
 class MCCVDefRangeFragment;
 class MCCVInlineLineTableFragment;
-class MCDataFragment;
 class MCFragment;
 class MCSection;
 class MCSymbol;
@@ -231,7 +230,7 @@ class CodeViewContext {
   StringMap<unsigned> StringTable;
 
   /// The fragment that ultimately holds our strings.
-  MCDataFragment *StrTabFragment = nullptr;
+  MCFragment *StrTabFragment = nullptr;
   SmallVector<char, 0> StrTab = {'\0'};
 
   /// Get a string table offset.

llvm/include/llvm/MC/MCContext.h

@@ -47,7 +47,6 @@ namespace llvm {
 
 class CodeViewContext;
 class MCAsmInfo;
-class MCDataFragment;
 class MCInst;
 class MCLabel;
 class MCObjectFileInfo;
@@ -334,7 +333,7 @@ class MCContext {
   void reportCommon(SMLoc Loc,
                     std::function<void(SMDiagnostic &, const SourceMgr *)>);
 
-  MCDataFragment *allocInitialFragment(MCSection &Sec);
+  MCFragment *allocInitialFragment(MCSection &Sec);
 
   MCSymbolTableEntry &getSymbolTableEntry(StringRef Name);
 

llvm/include/llvm/MC/MCELFStreamer.h

@@ -17,7 +17,6 @@ namespace llvm {
 
 class ELFObjectWriter;
 class MCContext;
-class MCDataFragment;
 class MCFragment;
 class MCObjectWriter;
 class MCSection;
@@ -51,7 +50,7 @@ class MCELFStreamer : public MCObjectStreamer {
   void initSections(bool NoExecStack, const MCSubtargetInfo &STI) override;
   void changeSection(MCSection *Section, uint32_t Subsection = 0) override;
   void emitLabel(MCSymbol *Symbol, SMLoc Loc = SMLoc()) override;
-  void emitLabelAtPos(MCSymbol *Symbol, SMLoc Loc, MCDataFragment &F,
+  void emitLabelAtPos(MCSymbol *Symbol, SMLoc Loc, MCFragment &F,
                       uint64_t Offset) override;
   void emitWeakReference(MCSymbol *Alias, const MCSymbol *Target) override;
   bool emitSymbolAttribute(MCSymbol *Symbol, MCSymbolAttr Attribute) override;

llvm/include/llvm/MC/MCObjectStreamer.h

@@ -43,8 +43,8 @@ class MCObjectStreamer : public MCStreamer {
   struct PendingMCFixup {
     const MCSymbol *Sym;
     MCFixup Fixup;
-    MCDataFragment *DF;
-    PendingMCFixup(const MCSymbol *McSym, MCDataFragment *F, MCFixup McFixup)
+    MCFragment *DF;
+    PendingMCFixup(const MCSymbol *McSym, MCFragment *F, MCFixup McFixup)
         : Sym(McSym), Fixup(McFixup), DF(F) {}
   };
   SmallVector<PendingMCFixup, 2> PendingFixups;
@@ -92,10 +92,10 @@ class MCObjectStreamer : public MCStreamer {
   }
 
   /// Get a data fragment to write into, creating a new one if the current
-  /// fragment is not a data fragment.
+  /// fragment is not FT_Data.
   /// Optionally a \p STI can be passed in so that a new fragment is created
   /// if the Subtarget differs from the current fragment.
-  MCDataFragment *getOrCreateDataFragment(const MCSubtargetInfo* STI = nullptr);
+  MCFragment *getOrCreateDataFragment(const MCSubtargetInfo *STI = nullptr);
 
 protected:
   bool changeSectionImpl(MCSection *Section, uint32_t Subsection);
@@ -109,7 +109,7 @@ class MCObjectStreamer : public MCStreamer {
   /// @{
 
   void emitLabel(MCSymbol *Symbol, SMLoc Loc = SMLoc()) override;
-  virtual void emitLabelAtPos(MCSymbol *Symbol, SMLoc Loc, MCDataFragment &F,
+  virtual void emitLabelAtPos(MCSymbol *Symbol, SMLoc Loc, MCFragment &F,
                               uint64_t Offset);
   void emitAssignment(MCSymbol *Symbol, const MCExpr *Value) override;
   void emitConditionalAssignment(MCSymbol *Symbol,