[Transforms] Remove unused declaration fillImplicitControlFlowInfo (NFC)

The definition was removed on Aug 7, 2018 in commit
640cb00, but the declaration has
remained since.

Author: kazutakahirata

llvm/include/llvm/Transforms/Scalar/GVN.h

@@ -338,7 +338,6 @@ class GVN : public PassInfoMixin<GVN> {
                                  BasicBlock *Curr, unsigned int ValNo);
   Value *findLeader(const BasicBlock *BB, uint32_t num);
   void cleanupGlobalSets();
-  void fillImplicitControlFlowInfo(BasicBlock *BB);
   void verifyRemoved(const Instruction *I) const;
   bool splitCriticalEdges();
   BasicBlock *splitCriticalEdges(BasicBlock *Pred, BasicBlock *Succ);