[DebugInfo] Map VAM args to poison instead of undef [NFC] (#122756)

If an argument cannot be mapped in `Mapper::mapValue`, it can be mapped
to `poison` instead of `undef`.

Author: pedroclobo

llvm/lib/Transforms/Utils/ValueMapper.cpp

@@ -410,9 +410,9 @@ Value *Mapper::mapValue(const Value *V) {
         } else if ((Flags & RF_IgnoreMissingLocals) && isa<LocalAsMetadata>(VAM)) {
             MappedArgs.push_back(VAM);
         } else {
-          // If we cannot map the value, set the argument as undef.
+          // If we cannot map the value, set the argument as poison.
           MappedArgs.push_back(ValueAsMetadata::get(
-              UndefValue::get(VAM->getValue()->getType())));
+              PoisonValue::get(VAM->getValue()->getType())));
         }
       }
       return MetadataAsValue::get(V->getContext(),

llvm/unittests/Transforms/Utils/ValueMapperTest.cpp

@@ -347,8 +347,8 @@ TEST(ValueMapperTest, mapValueLocalInArgList) {
   // such as "metadata i32 %x" don't currently successfully maintain that
   // property.  To keep RemapInstruction from crashing we need a non-null
   // return here, but we also shouldn't reference the unmapped local.  Use
-  // undef for uses in a DIArgList.
-  auto *N0 = UndefValue::get(Type::getInt8Ty(C));
+  // poison for uses in a DIArgList.
+  auto *N0 = PoisonValue::get(Type::getInt8Ty(C));
   auto *N0AM = ValueAsMetadata::get(N0);
   std::vector<ValueAsMetadata*> N0Elts;
   N0Elts.push_back(N0AM);