[Constant] Make Constant::getSplatValue return poison on poison

This is a follow up from llvm#141845. I'm not sure if this actually NFC but it doesn't seem to affect any of the in-tree tests.

I went through the users of getSplatValue to see if anything could be cleaned up but nothing immediately stuck out.

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/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))