[SCEV] Remove unused getExactExistingExpansion (NFC)

The last use was removed on Sep 4, 2018 in commit
2cbba56.

Author: kazutakahirata

llvm/include/llvm/Transforms/Utils/ScalarEvolutionExpander.h

@@ -365,10 +365,6 @@ class SCEVExpander : public SCEVVisitor<SCEVExpander, Value *> {
 
   void setChainedPhi(PHINode *PN) { ChainedPhis.insert(PN); }
 
-  /// Try to find existing LLVM IR value for S available at the point At.
-  Value *getExactExistingExpansion(const SCEV *S, const Instruction *At,
-                                   Loop *L);
-
   /// Try to find the ValueOffsetPair for S. The function is mainly used to
   /// check whether S can be expanded cheaply.  If this returns a non-None
   /// value, we know we can codegen the `ValueOffsetPair` into a suitable

llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp

@@ -2152,15 +2152,6 @@ SCEVExpander::replaceCongruentIVs(Loop *L, const DominatorTree *DT,
   return NumElim;
 }
 
-Value *SCEVExpander::getExactExistingExpansion(const SCEV *S,
-                                               const Instruction *At, Loop *L) {
-  Optional<ScalarEvolution::ValueOffsetPair> VO =
-      getRelatedExistingExpansion(S, At, L);
-  if (VO && VO.getValue().second == nullptr)
-    return VO.getValue().first;
-  return nullptr;
-}
-
 Optional<ScalarEvolution::ValueOffsetPair>
 SCEVExpander::getRelatedExistingExpansion(const SCEV *S, const Instruction *At,
                                           Loop *L) {