[VPlan] Use start value of reduction phi to determine type (NFCI).

Instead of accessing the underlying original IR value, check the type of
the start value from the recipe directly.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -1712,16 +1712,20 @@ void VPFirstOrderRecurrencePHIRecipe::print(raw_ostream &O, const Twine &Indent,
 #endif
 
 void VPReductionPHIRecipe::execute(VPTransformState &State) {
-  PHINode *PN = cast<PHINode>(getUnderlyingValue());
   auto &Builder = State.Builder;
 
+  // Reductions do not have to start at zero. They can start with
+  // any loop invariant values.
+  VPValue *StartVPV = getStartValue();
+  Value *StartV = StartVPV->getLiveInIRValue();
+
   // In order to support recurrences we need to be able to vectorize Phi nodes.
   // Phi nodes have cycles, so we need to vectorize them in two stages. This is
   // stage #1: We create a new vector PHI node with no incoming edges. We'll use
   // this value when we vectorize all of the instructions that use the PHI.
   bool ScalarPHI = State.VF.isScalar() || IsInLoop;
-  Type *VecTy =
-      ScalarPHI ? PN->getType() : VectorType::get(PN->getType(), State.VF);
+  Type *VecTy = ScalarPHI ? StartV->getType()
+                          : VectorType::get(StartV->getType(), State.VF);
 
   BasicBlock *HeaderBB = State.CFG.PrevBB;
   assert(State.CurrentVectorLoop->getHeader() == HeaderBB &&
@@ -1735,11 +1739,6 @@ void VPReductionPHIRecipe::execute(VPTransformState &State) {
 
   BasicBlock *VectorPH = State.CFG.getPreheaderBBFor(this);
 
-  // Reductions do not have to start at zero. They can start with
-  // any loop invariant values.
-  VPValue *StartVPV = getStartValue();
-  Value *StartV = StartVPV->getLiveInIRValue();
-
   Value *Iden = nullptr;
   RecurKind RK = RdxDesc.getRecurrenceKind();
   if (RecurrenceDescriptor::isMinMaxRecurrenceKind(RK) ||