[ValueMapper] allow mapping ConstantTargetNone to its layout type

[wenju]

Summary

zeroinitializer is allowed for spirv TargetExtType.
This PR allows ValueMapper to map TargetExtType ConstantTargetNone to '0' constant of its layout type.

llvm/lib/Transforms/Utils/ValueMapper.cpp

if (isa<ConstantVector>(C))
     return getVM()[V] = ConstantVector::get(Ops);
   // If this is a no-operand constant, it must be because the type was remapped.
   if (isa<PoisonValue>(C))
     return getVM()[V] = PoisonValue::get(NewTy);
   if (isa<UndefValue>(C))
     return getVM()[V] = UndefValue::get(NewTy);
   if (isa<ConstantAggregateZero>(C))
     return getVM()[V] = ConstantAggregateZero::get(NewTy);
+  if (isa<ConstantTargetNone>(C))
+    return getVM()[V] = Constant::getNullValue(NewTy);
   assert(isa<ConstantPointerNull>(C));
   return getVM()[V] = ConstantPointerNull::get(cast<PointerType>(NewTy));
 }
 
 Value *Mapper::mapBlockAddress(const BlockAddress &BA) {
   Function *F = cast<Function>(mapValue(BA.getFunction()));
 
   // F may not have materialized its initializer.  In that case, create a

llvm/unittests/Transforms/Utils/ValueMapperTest.cpp

TEST(ValueMapperTest, mapValuePoisonWithTypeRemap) {
   ValueMapper Mapper(VM, RF_None, &TM);
 
   // Check that poison is still poison and has not been converted to undef.
   auto *OldPoison = PoisonValue::get(OldTy);
   auto *NewPoison = PoisonValue::get(NewTy);
   EXPECT_EQ(NewPoison, Mapper.mapValue(*OldPoison));
 }
 
+TEST(ValueMapperTest, mapValueConstantTargetNoneToLayoutTypeNullValue) {
+  LLVMContext C;
+  auto *OldTy = TargetExtType::get(C, "spirv.Image");
+  Type *NewTy = OldTy->getLayoutType();
+
+  TestTypeRemapper TM(NewTy);
+  ValueToValueMapTy VM;
+  ValueMapper Mapper(VM, RF_None, &TM);
+
+  // Check that ConstantTargetNone is mapped to '0' constant of its layout type.
+  auto *OldConstant = ConstantTargetNone::get(OldTy);
+  auto *NewConstant = Constant::getNullValue(NewTy);
+  EXPECT_EQ(NewConstant, Mapper.mapValue(*OldConstant));
+}
+
 } // end namespace