[ConstantFold] Make areGlobalsPotentiallyEqual less aggressive.

In particular, we shouldn't make assumptions about globals which are
unnamed_addr: we can fold them together with other globals.

Also while I'm here, use isInterposable() instead of trying to
explicitly name all the different kinds of weak linkage.

Fixes https://bugs.llvm.org/show_bug.cgi?id=47090

Differential Revision: https://reviews.llvm.org/D87123

(cherry picked from commit d751f86)

llvm/lib/IR/ConstantFold.cpp

@@ -1589,7 +1589,7 @@ static FCmpInst::Predicate evaluateFCmpRelation(Constant *V1, Constant *V2) {
 static ICmpInst::Predicate areGlobalsPotentiallyEqual(const GlobalValue *GV1,
                                                       const GlobalValue *GV2) {
   auto isGlobalUnsafeForEquality = [](const GlobalValue *GV) {
-    if (GV->hasExternalWeakLinkage() || GV->hasWeakAnyLinkage())
+    if (GV->isInterposable() || GV->hasGlobalUnnamedAddr())
       return true;
     if (const auto *GVar = dyn_cast<GlobalVariable>(GV)) {
       Type *Ty = GVar->getValueType();

llvm/test/Assembler/ConstantExprNoFold.ll

@@ -42,6 +42,12 @@ target datalayout = "p:32:32"
 @empty.2 = external global [0 x i8], align 1
 @empty.cmp = global i1 icmp eq ([0 x i8]* @empty.1, [0 x i8]* @empty.2)
 
+; Two unnamed_addr globals can share an address
+; CHECK: @unnamed.cmp = global i1 icmp eq ([5 x i8]* @unnamed.1, [5 x i8]* @unnamed.2)
+@unnamed.1 = unnamed_addr constant [5 x i8] c"asdf\00"
+@unnamed.2 = unnamed_addr constant [5 x i8] c"asdf\00"
+@unnamed.cmp = global i1 icmp eq ([5 x i8]* @unnamed.1, [5 x i8]* @unnamed.2)
+
 @addrspace3 = internal addrspace(3) global i32 undef
 
 ; CHECK: @no.fold.addrspace.icmp.eq.gv.null = global i1 icmp eq (i32 addrspace(3)* @addrspace3, i32 addrspace(3)* null)

llvm/test/Transforms/InstCombine/2010-03-03-ExtElim.ll

@@ -16,8 +16,8 @@ define i1 @PR6486() nounwind {
 ; CHECK: ret i1 true
 }
 
-@d = common global i32 0, align 4
-@a = common global [1 x i32] zeroinitializer, align 4
+@d = global i32 0, align 4
+@a = global [1 x i32] zeroinitializer, align 4
 
 define i1 @PR16462_1() nounwind {
 ; CHECK-LABEL: @PR16462_1(