Update comments to clarify which parts can only be used in the vplan-native path, also add back the assertion but check that the incomingblocks is empty

Author: lukel97

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -2318,13 +2318,15 @@ class VPWidenPHIRecipe : public VPSingleDefRecipe {
              VPSlotTracker &SlotTracker) const override;
 #endif
 
-  /// Adds a pair (\p IncomingV, \p IncomingBlock) to the phi.
+  /// Adds a pair (\p IncomingV, \p IncomingBlock) to the phi. Only used in the
+  /// VPlan native path.
   void addIncoming(VPValue *IncomingV, VPBasicBlock *IncomingBlock) {
     addOperand(IncomingV);
     IncomingBlocks.push_back(IncomingBlock);
   }
 
-  /// Returns the \p I th incoming VPBasicBlock.
+  /// Returns the \p I th incoming VPBasicBlock. Only used in the VPlan native
+  /// path.
   VPBasicBlock *getIncomingBlock(unsigned I) { return IncomingBlocks[I]; }
 
   /// Returns the \p I th incoming VPValue.

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -3440,6 +3440,10 @@ void VPReductionPHIRecipe::print(raw_ostream &O, const Twine &Indent,
 #endif
 
 void VPWidenPHIRecipe::execute(VPTransformState &State) {
+  assert((EnableVPlanNativePath || IncomingBlocks.empty()) &&
+         "Non-native vplans are not expected to have VPWidenPHIRecipes with "
+         "incoming blocks.");
+
   State.setDebugLocFrom(getDebugLoc());
   Value *Op0 = State.get(getOperand(0));
   Type *VecTy = Op0->getType();