[VPlan] Make Phi operand for VPReductionPHIRecipe optional (NFC).

VPReductionPHIRecipe doesn't rely on the underlying phi any longer,
allow empty underlying values when cloning. NFC at the moment but will
enable follow-up patches.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -2212,9 +2212,9 @@ class VPReductionPHIRecipe : public VPHeaderPHIRecipe,
   ~VPReductionPHIRecipe() override = default;
 
   VPReductionPHIRecipe *clone() override {
-    auto *R = new VPReductionPHIRecipe(cast<PHINode>(getUnderlyingInstr()),
-                                       RdxDesc, *getOperand(0), IsInLoop,
-                                       IsOrdered, VFScaleFactor);
+    auto *R = new VPReductionPHIRecipe(
+        dyn_cast_or_null<PHINode>(getUnderlyingValue()), RdxDesc,
+        *getOperand(0), IsInLoop, IsOrdered, VFScaleFactor);
     R->addOperand(getBackedgeValue());
     return R;
   }