[DebugInfo] Replace UndefValue with PoisonValue in DIArgList::handleChangedOperand

[Orlando]

Summary

This helps towards the effort to remove UndefValue from LLVM.

Related to https://discourse.llvm.org/t/auto-undef-debug-uses-of-a-deleted-value

[nlopes]

LGTM, thanks!

llvm/lib/IR/DebugInfoMetadata.cpp

if (Uniq) {
     eraseFromStore();
   }
   ValueAsMetadata *NewVM = cast_or_null<ValueAsMetadata>(New);
   for (ValueAsMetadata *&VM : Args) {
     if (&VM == OldVMPtr) {
       if (NewVM)
         VM = NewVM;
       else
-        VM = ValueAsMetadata::get(UndefValue::get(VM->getValue()->getType()));
+        VM = ValueAsMetadata::get(PoisonValue::get(VM->getValue()->getType()));
     }
   }
   if (Uniq) {
     if (uniquify() != this)
       storeDistinctInContext();
   }
   track();
 }

llvm/test/Transforms/GlobalOpt/deadglobal-diarglist-use.ll

 ; RUN: opt < %s -passes=globalopt -S | llvm-as | llvm-dis | FileCheck %s
 
 ; The %struct.S type would not get emitted after @s was removed, resulting in
 ; llvm-as failing to parse the dbg.value intrinsic using that type. However,
 ; with opaque pointers this is no longer necessary.
 
 ; CHECK-NOT: @s
 
-; CHECK: call void @llvm.dbg.value(metadata !DIArgList(ptr undef
+; CHECK: call void @llvm.dbg.value(metadata !DIArgList(ptr poison
 
 %struct.S = type { i32 }
 
 @s = internal global [1 x %struct.S] zeroinitializer, align 4, !dbg !0
 @idx = dso_local global i32 0, align 4, !dbg !6
 
 define dso_local void @fn() !dbg !20 {
 entry: