…long encodings (llvm#205907)

When a (signed or unsigned) LEB128 value is encoded with extra trailing
bytes that only carry zero- or sign-extension, the decode loop could
keep
running with the shift amount at 64 or beyond and then evaluate
`Slice << Shift`, which is undefined behavior for a 64-bit type.

The DWARF expression parser feeds attacker-controlled LEB128 operands
(such as `DW_OP_bregN` / `DW_OP_constu`) through
`DataExtractor::getULEB128` / `getSLEB128`, so the
`lldb-dwarf-expression-fuzzer` reaches this under UBSan.  The unsigned
case:

```
LEB128.h:152:20: runtime error: shift exponent 70 is too large for
64-bit type 'uint64_t' (aka 'unsigned long long')
    #0 llvm::decodeULEB128(...) LEB128.h:152
    #1 getLEB128<unsigned long long>(...) DataExtractor.cpp:227
    #2 llvm::DataExtractor::getULEB128(...) DataExtractor.cpp:241
    #3 llvm::DWARFExpression::Operation::extract(...) DWARFExpression.cpp:218
    llvm#7 lldb_private::DWARFExpression::Evaluate(...) DWARFExpression.cpp:1333
    llvm#9 LLVMFuzzerTestOneInput lldb-dwarf-expression-fuzzer.cpp:83
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior LEB128.h:152:20
```

and the signed case, reached the same way:

```
LEB128.h:190:20: runtime error: shift exponent 70 is too large for
64-bit type 'uint64_t' (aka 'unsigned long long')
    #0 llvm::decodeSLEB128(...) LEB128.h:190
    #1 getLEB128<long long>(...) DataExtractor.cpp:227
    #2 llvm::DataExtractor::getSLEB128(...) DataExtractor.cpp:245
    #3 llvm::DWARFExpression::Operation::extract(...) DWARFExpression.cpp:216
    llvm#7 lldb_private::DWARFExpression::Evaluate(...) DWARFExpression.cpp:1333
```

The existing range checks already guarantee that once `Shift` reaches 64
the remaining bytes are pure extension and contribute nothing to the
result, so skip the accumulating shift in that case.  Decoded values are
unchanged for all well-formed inputs.

Adds overlong-encoding regression cases to `LEB128Test`. Without the fix
the signed case is the UBSan diagnostic above in a sanitizer build, and
in a normal build the unsigned case also decodes to the wrong value (the
overlong `1` decodes as `11`).

…lvm#212759)

I see the following 

```
 anutosh491@Anutoshs-MacBook-Air llvm-project % cat /private/tmp/wasm-scalable-shift-cost.ll
define <vscale x 4 x i32> @shift(<vscale x 4 x i32> %x,
                                 <vscale x 4 x i32> %amount) {
  %result = shl <vscale x 4 x i32> %x, %amount
  ret <vscale x 4 x i32> %result
}


 anutosh491@Anutoshs-MacBook-Air llvm-project % build-assert/bin/opt \
  -mtriple=wasm32-unknown-unknown \
  -mattr=+simd128 \
  -passes='print<cost-model>' \
  -disable-output \
  /private/tmp/wasm-scalable-shift-cost.ll
Printing analysis 'Cost Model Analysis' for function 'shift':
Cost Model: Assertion failed: (isa<To>(Val) && "cast<Ty>() argument of incompatible type!"), function cast, file Casting.h, line 572.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace and instructions to reproduce the bug.
Stack dump:
0.      Program arguments: build-assert/bin/opt -mtriple=wasm32-unknown-unknown -mattr=+simd128 -passes=print<cost-model> -disable-output /private/tmp/wasm-scalable-shift-cost.ll
1.      Running pass "function(print<cost-model>)" on module "/private/tmp/wasm-scalable-shift-cost.ll"
2.      Running pass "print<cost-model>" on function "shift"
 #0 0x0000000105b7a56c llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/Users/anutosh491/work/llvm-project/build-assert/bin/opt+0x102f6256c)
 #1 0x0000000105b78248 llvm::sys::RunSignalHandlers() (/Users/anutosh491/work/llvm-project/build-assert/bin/opt+0x102f60248)
 #2 0x0000000105b7b170 SignalHandler(int, __siginfo*, void*) (/Users/anutosh491/work/llvm-project/build-assert/bin/opt+0x102f63170)
 #3 0x000000018b5f7744 (/usr/lib/system/libsystem_platform.dylib+0x1804fb744)
 #4 0x000000018b5ed8d8 (/usr/lib/system/libsystem_pthread.dylib+0x1804f18d8)
 #5 0x000000018b4f4644 (/usr/lib/system/libsystem_c.dylib+0x1803f8644)
 llvm#6 0x000000018b4f38a0 (/usr/lib/system/libsystem_c.dylib+0x1803f78a0)
 llvm#7 0x0000000104036ddc llvm::BasicTTIImplBase<llvm::WebAssemblyTTIImpl>::getArithmeticInstrCost(unsigned int, llvm::Type*, llvm::TargetTransformInfo::TargetCostKind, llvm::TargetTransformInfo::OperandValueInfo, llvm::TargetTransformInfo::OperandValueInfo, llvm::ArrayRef<llvm::Value const*>, llvm::Instruction const*) const (/Users/anutosh491/work/llvm-project/build-assert/bin/opt+0x10141eddc)
 llvm#8 0x000000010403b0ac llvm::TargetTransformInfoImplCRTPBase<llvm::WebAssemblyTTIImpl>::getInstructionCost(llvm::User const*, llvm::ArrayRef<llvm::Value const*>, llvm::TargetTransformInfo::TargetCostKind) const (/Users/anutosh491/work/llvm-project/build-assert/bin/opt+0x1014230ac)
 llvm#9 0x00000001049162c8 llvm::TargetTransformInfo::getInstructionCost(llvm::User const*, llvm::ArrayRef<llvm::Value const*>, llvm::TargetTransformInfo::TargetCostKind) const (/Users/anutosh491/work/llvm-project/build-assert/bin/opt+0x101cfe2c8)
llvm#10 0x00000001058d90b4 llvm::TargetTransformInfo::getInstructionCost(llvm::User const*, llvm::TargetTransformInfo::TargetCostKind) const (/Users/anutosh491/work/llvm-project/build-assert/bin/opt+0x102cc10b4)
llvm#11 0x000000010468a368 getCost(llvm::Instruction&, llvm::TargetTransformInfo::TargetCostKind, llvm::TargetTransformInfo&) (/Users/anutosh491/work/llvm-project/build-assert/bin/opt+0x101a72368)
llvm#12 0x0000000104689dd4 llvm::CostModelPrinterPass::run(llvm::Function&, llvm::AnalysisManager<llvm::Function>&) (/Users/anutosh491/work/llvm-project/build-assert/bin/opt+0x101a71dd4)
llvm#13 0x0000000105186554 llvm::PassManager<llvm::Function, llvm::AnalysisManager<llvm::Function>>::run(llvm::Function&, llvm::AnalysisManager<llvm::Function>&) (/Users/anutosh491/work/llvm-project/build-assert/bin/opt+0x10256e554)
llvm#14 0x000000010518a680 llvm::ModuleToFunctionPassAdaptor::run(llvm::Module&, llvm::AnalysisManager<llvm::Module>&) (/Users/anutosh491/work/llvm-project/build-assert/bin/opt+0x102572680)
llvm#15 0x0000000105185418 llvm::PassManager<llvm::Module, llvm::AnalysisManager<llvm::Module>>::run(llvm::Module&, llvm::AnalysisManager<llvm::Module>&) (/Users/anutosh491/work/llvm-project/build-assert/bin/opt+0x10256d418)
llvm#16 0x0000000106127cb0 llvm::runPassPipeline(llvm::StringRef, llvm::Module&, llvm::TargetMachine*, llvm::TargetLibraryInfoImpl*, llvm::ToolOutputFile*, llvm::ToolOutputFile*, llvm::ToolOutputFile*, llvm::StringRef, llvm::ArrayRef<llvm::PassPlugin>, llvm::ArrayRef<std::__1::function<void (llvm::PassBuilder&)>>, llvm::opt_tool::OutputKind, llvm::opt_tool::VerifierKind, bool, bool, bool, bool, bool, bool, bool, bool) (/Users/anutosh491/work/llvm-project/build-assert/bin/opt+0x10350fcb0)
llvm#17 0x0000000106131eac optMain (/Users/anutosh491/work/llvm-project/build-assert/bin/opt+0x103519eac)
llvm#18 0x000000018b22fe00
zsh: abort      build-assert/bin/opt -mtriple=wasm32-unknown-unknown -mattr=+simd128 
```