[llvm] [AMDGPU] Fold llvm.amdgcn.cvt.pkrtz when either operand is fpext (PR #108237)

[Jay Foad via llvm-commits]

================
@@ -643,27 +643,38 @@ GCNTTIImpl::instCombineIntrinsic(InstCombiner &IC, IntrinsicInst &II) const {
     break;
   }
   case Intrinsic::amdgcn_cvt_pkrtz: {
-    Value *Src0 = II.getArgOperand(0);
-    Value *Src1 = II.getArgOperand(1);
-    if (const ConstantFP *C0 = dyn_cast<ConstantFP>(Src0)) {
-      if (const ConstantFP *C1 = dyn_cast<ConstantFP>(Src1)) {
-        const fltSemantics &HalfSem =
-            II.getType()->getScalarType()->getFltSemantics();
+    auto foldFPTruncToF16RTZ = [](Value *Arg) -> Value * {
+      Type *HalfTy = Type::getHalfTy(Arg->getContext());
+
+      if (isa<PoisonValue>(Arg))
+        return PoisonValue::get(HalfTy);
+      if (isa<UndefValue>(Arg))
+        return UndefValue::get(HalfTy);
----------------
jayfoad wrote:

I was following [ConstantFoldCastInstruction](https://github.com/llvm/llvm-project/blob/7294396a0878a6bd179fac9aa5c3743832c799f4/llvm/lib/IR/ConstantFold.cpp#L142) which will fold (fptrunc undef) to undef.

https://github.com/llvm/llvm-project/pull/108237