[VPlan] Mark VPlan argument in isHeaderMask as const (NFC).

isHeaderMask should not modify the VPlan; mark as const to allow easy
re-use in the VPlanVerifier.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -4267,12 +4267,14 @@ class VPlan {
       BackedgeTakenCount = new VPValue();
     return BackedgeTakenCount;
   }
+  VPValue *getBackedgeTakenCount() const { return BackedgeTakenCount; }
 
   /// The vector trip count.
   VPValue &getVectorTripCount() { return VectorTripCount; }
 
   /// Returns the VF of the vector loop region.
   VPValue &getVF() { return VF; };
+  const VPValue &getVF() const { return VF; };
 
   /// Returns VF * UF of the vector loop region.
   VPValue &getVFxUF() { return VFxUF; }

llvm/lib/Transforms/Vectorize/VPlanUtils.cpp

@@ -53,7 +53,7 @@ VPValue *vputils::getOrCreateVPValueForSCEVExpr(VPlan &Plan, const SCEV *Expr) {
   return Expanded;
 }
 
-bool vputils::isHeaderMask(const VPValue *V, VPlan &Plan) {
+bool vputils::isHeaderMask(const VPValue *V, const VPlan &Plan) {
   if (isa<VPActiveLaneMaskPHIRecipe>(V))
     return true;
 
@@ -74,7 +74,7 @@ bool vputils::isHeaderMask(const VPValue *V, VPlan &Plan) {
             IsWideCanonicalIV(A));
 
   return match(V, m_ICmp(m_VPValue(A), m_VPValue(B))) && IsWideCanonicalIV(A) &&
-         B == Plan.getOrCreateBackedgeTakenCount();
+         B == Plan.getBackedgeTakenCount();
 }
 
 const SCEV *vputils::getSCEVExprForVPValue(VPValue *V, ScalarEvolution &SE) {

llvm/lib/Transforms/Vectorize/VPlanUtils.h

@@ -90,7 +90,7 @@ inline bool isSingleScalar(const VPValue *VPV) {
 }
 
 /// Return true if \p V is a header mask in \p Plan.
-bool isHeaderMask(const VPValue *V, VPlan &Plan);
+bool isHeaderMask(const VPValue *V, const VPlan &Plan);
 
 /// Checks if \p V is uniform across all VF lanes and UF parts. It is considered
 /// as such if it is either loop invariant (defined outside the vector region)