[LV] Fixup IV users only once during epilogue vectorization. (NFC)

Induction users only need to be updated when vectorizing the epilogue.
Avoid running fixupIVUsers when vectorizing the main loop during
epilogue vectorization.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -539,10 +539,10 @@ class InnerLoopVectorizer {
   friend class LoopVectorizationPlanner;
 
   /// Set up the values of the IVs correctly when exiting the vector loop.
-  void fixupIVUsers(PHINode *OrigPhi, const InductionDescriptor &II,
-                    Value *VectorTripCount, Value *EndValue,
-                    BasicBlock *MiddleBlock, VPlan &Plan,
-                    VPTransformState &State);
+  virtual void fixupIVUsers(PHINode *OrigPhi, const InductionDescriptor &II,
+                            Value *VectorTripCount, Value *EndValue,
+                            BasicBlock *MiddleBlock, VPlan &Plan,
+                            VPTransformState &State);
 
   /// Iteratively sink the scalarized operands of a predicated instruction into
   /// the block that was created for it.
@@ -770,6 +770,11 @@ class EpilogueVectorizerMainLoop : public InnerLoopAndEpilogueVectorizer {
   BasicBlock *emitIterationCountCheck(BasicBlock *Bypass, bool ForEpilogue);
   void printDebugTracesAtStart() override;
   void printDebugTracesAtEnd() override;
+
+  void fixupIVUsers(PHINode *OrigPhi, const InductionDescriptor &II,
+                    Value *VectorTripCount, Value *EndValue,
+                    BasicBlock *MiddleBlock, VPlan &Plan,
+                    VPTransformState &State) override {};
 };
 
 // A specialized derived class of inner loop vectorizer that performs