CodeGen: Allow !associated metadata to point to aliases.

[pcc]

Summary

This is a small extension of !associated, mostly useful for the implementation
convenience of instrumentation passes that RAUW globals with aliases, such
as LowerTypeTests.

[eugenis]

LGTM

llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

static const MCSymbolELF *getAssociatedSymbol(const GlobalObject *GO,
   const MDOperand &Op = MD->getOperand(0);
   if (!Op.get())
     return nullptr;
 
   auto *VM = dyn_cast<ValueAsMetadata>(Op);
   if (!VM)
     report_fatal_error("MD_associated operand is not ValueAsMetadata");
 
-  GlobalObject *OtherGO = dyn_cast<GlobalObject>(VM->getValue());
-  return OtherGO ? dyn_cast<MCSymbolELF>(TM.getSymbol(OtherGO)) : nullptr;
+  auto *OtherGV = dyn_cast<GlobalValue>(VM->getValue());
+  return OtherGV ? dyn_cast<MCSymbolELF>(TM.getSymbol(OtherGV)) : nullptr;
 }
 
 static unsigned getEntrySizeForKind(SectionKind Kind) {
   if (Kind.isMergeable1ByteCString())
     return 1;
   else if (Kind.isMergeable2ByteCString())
     return 2;
   else if (Kind.isMergeable4ByteCString())

llvm/test/CodeGen/X86/elf-associated.ll

 @j = global i32 1, section "bbb", !associated !4
 @k = global i32 1, !associated !4
 !4 = !{i32* @h}
 ; CHECK-DAG: .section aaa,"aw",@progbits
 ; CHECK-DAG: .section bbb,"awo",@progbits,h,unique,1
 ; CHECK-DAG: .section bbb,"awo",@progbits,h,unique,2
 ; CHECK-DAG: .section .data.k,"awo",@progbits,h
 
-; Non-GlobalObject metadata.
+; Non-GlobalValue metadata.
 @l = global i32 1, section "ccc", !associated !5
 !5 = !{i32* null}
 ; CHECK-DAG: .section ccc,"aw",@progbits
 
 ; Null metadata.
 @m = global i32 1, section "ddd", !associated !6
 !6 = distinct !{null}
 ; CHECK-DAG: .section ddd,"aw",@progbits
+
+; Aliases are OK.
+@n = alias i32, i32* inttoptr (i64 add (i64 ptrtoint (i32* @a to i64), i64 1297036692682702848) to i32*)
+@o = global i32 1, section "eee", !associated !7
+!7 = !{i32* @n}
+; CHECK-DAG: .section eee,"awo",@progbits,n,unique,3