[Constant] Make Constant::getSplatValue return poison on poison (#141870)

This is a follow up from #141845. 

TargetTransformInfo::getOperandInfo needs to be updated to check for
undef values as otherwise a splat is considered a constant, and some
RISC-V cost model tests will start adding a cost to materialize the
constant.

Author: lukel97

llvm/lib/Analysis/ConstantFolding.cpp

@@ -3794,11 +3794,6 @@ static Constant *ConstantFoldScalableVectorCall(
       SplatOps.push_back(Op);
       continue;
     }
-    // TODO: Should getSplatValue return a poison scalar for a poison vector?
-    if (isa<PoisonValue>(Op)) {
-      SplatOps.push_back(PoisonValue::get(Op->getType()->getScalarType()));
-      continue;
-    }
     Constant *Splat = Op->getSplatValue();
     if (!Splat)
       return nullptr;

llvm/lib/Analysis/TargetTransformInfo.cpp

@@ -886,6 +886,10 @@ TargetTransformInfo::getOperandInfo(const Value *V) {
   OperandValueKind OpInfo = OK_AnyValue;
   OperandValueProperties OpProps = OP_None;
 
+  // undef/poison don't materialize constants.
+  if (isa<UndefValue>(V))
+    return {OK_AnyValue, OP_None};
+
   if (isa<ConstantInt>(V) || isa<ConstantFP>(V)) {
     if (const auto *CI = dyn_cast<ConstantInt>(V)) {
       if (CI->getValue().isPowerOf2())

llvm/lib/IR/Constants.cpp

@@ -1711,6 +1711,8 @@ void ConstantVector::destroyConstantImpl() {
 
 Constant *Constant::getSplatValue(bool AllowPoison) const {
   assert(this->getType()->isVectorTy() && "Only valid for vectors!");
+  if (isa<PoisonValue>(this))
+    return PoisonValue::get(cast<VectorType>(getType())->getElementType());
   if (isa<ConstantAggregateZero>(this))
     return getNullValue(cast<VectorType>(getType())->getElementType());
   if (auto *CI = dyn_cast<ConstantInt>(this))