[VPlan] Update canNarrowLoad to check WidenMember0's op first (NFCI).

This hardens the code to check based on WideMember0's operands. This
ensures each call will go through the same check. Should be NFC
currently but needed when generalizing in follow-up patches.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -4147,13 +4147,13 @@ VPlanTransforms::expandSCEVs(VPlan &Plan, ScalarEvolution &SE) {
 /// is defined at \p Idx of a load interleave group.
 static bool canNarrowLoad(VPWidenRecipe *WideMember0, unsigned OpIdx,
                           VPValue *OpV, unsigned Idx) {
-  auto *DefR = OpV->getDefiningRecipe();
-  if (!DefR)
-    return WideMember0->getOperand(OpIdx) == OpV;
-  if (auto *W = dyn_cast<VPWidenLoadRecipe>(DefR))
-    return !W->getMask() && WideMember0->getOperand(OpIdx) == OpV;
-
-  if (auto *IR = dyn_cast<VPInterleaveRecipe>(DefR))
+  VPValue *Member0Op = WideMember0->getOperand(OpIdx);
+  VPRecipeBase *Member0OpR = Member0Op->getDefiningRecipe();
+  if (!Member0OpR)
+    return Member0Op == OpV;
+  if (auto *W = dyn_cast<VPWidenLoadRecipe>(Member0OpR))
+    return !W->getMask() && Member0Op == OpV;
+  if (auto *IR = dyn_cast<VPInterleaveRecipe>(Member0OpR))
     return IR->getInterleaveGroup()->isFull() && IR->getVPValue(Idx) == OpV;
   return false;
 }