[VPlan] Handle canonical IVs in ::isSingleScalar. (NFCI)

The canonical IV is always a single scalar. They are already treated as
uniform-across-UF-and-VF.

This should currently be NFC.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlanUtils.cpp

@@ -197,7 +197,8 @@ bool vputils::isSingleScalar(const VPValue *VPV) {
             all_of(VPI->operands(), isSingleScalar));
   if (auto *RR = dyn_cast<VPReductionRecipe>(VPV))
     return !RR->isPartialReduction();
-  if (isa<VPVectorPointerRecipe, VPVectorEndPointerRecipe>(VPV))
+  if (isa<VPCanonicalIVPHIRecipe, VPVectorPointerRecipe,
+          VPVectorEndPointerRecipe>(VPV))
     return true;
   if (auto *Expr = dyn_cast<VPExpressionRecipe>(VPV))
     return Expr->isSingleScalar();