[VPlan] Make ExpandedSCEVs argument const (NFC).

The argument is only used in const contexts. Simplifies a follow-up
diff.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorizationPlanner.h

@@ -354,7 +354,7 @@ class LoopVectorizationPlanner {
   executePlan(ElementCount VF, unsigned UF, VPlan &BestPlan,
               InnerLoopVectorizer &LB, DominatorTree *DT,
               bool IsEpilogueVectorization,
-              DenseMap<const SCEV *, Value *> *ExpandedSCEVs = nullptr);
+              const DenseMap<const SCEV *, Value *> *ExpandedSCEVs = nullptr);
 
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
   void printPlans(raw_ostream &O);

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7644,7 +7644,7 @@ static void AddRuntimeUnrollDisableMetaData(Loop *L) {
 SCEV2ValueTy LoopVectorizationPlanner::executePlan(
     ElementCount BestVF, unsigned BestUF, VPlan &BestVPlan,
     InnerLoopVectorizer &ILV, DominatorTree *DT, bool IsEpilogueVectorization,
-    DenseMap<const SCEV *, Value *> *ExpandedSCEVs) {
+    const DenseMap<const SCEV *, Value *> *ExpandedSCEVs) {
   assert(BestVPlan.hasVF(BestVF) &&
          "Trying to execute plan with unsupported VF");
   assert(BestVPlan.hasUF(BestUF) &&