[VPlan] Mark canonical IV and reduction phis as not writing memory (NFC).

Both recipes do not write to memory. Should be NFC at the moment, as
they cannot be removed currently due to being in a cycle.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -67,8 +67,10 @@ bool VPRecipeBase::mayWriteToMemory() const {
                 ->onlyReadsMemory();
   case VPWidenIntrinsicSC:
     return cast<VPWidenIntrinsicRecipe>(this)->mayWriteToMemory();
+  case VPCanonicalIVPHISC:
   case VPBranchOnMaskSC:
   case VPFirstOrderRecurrencePHISC:
+  case VPReductionPHISC:
   case VPScalarIVStepsSC:
   case VPPredInstPHISC:
     return false;