[MemorySSA] Remove unused declaration findDominatingDef (NFC)

The function definition was removed on Feb 22, 2017 in commit
17e8d0e.  The declaration has
remained since.

llvm/include/llvm/Analysis/MemorySSA.h

@@ -857,7 +857,6 @@ class MemorySSA {
   template <typename AliasAnalysisType>
   MemoryUseOrDef *createNewAccess(Instruction *, AliasAnalysisType *,
                                   const MemoryUseOrDef *Template = nullptr);
-  MemoryAccess *findDominatingDef(BasicBlock *, enum InsertionPlace);
   void placePHINodes(const SmallPtrSetImpl<BasicBlock *> &);
   MemoryAccess *renameBlock(BasicBlock *, MemoryAccess *, bool);
   void renameSuccessorPhis(BasicBlock *, MemoryAccess *, bool);