Revert change r12912 due to x64 perf issue

src/x64/lithium-codegen-x64.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 323 matching lines @@
     Address address;
   };
 
   void EnsureSpaceForLazyDeopt(int space_needed);
   void DoLoadKeyedExternalArray(LLoadKeyed* instr);
   void DoLoadKeyedFixedDoubleArray(LLoadKeyed* instr);
   void DoLoadKeyedFixedArray(LLoadKeyed* instr);
   void DoStoreKeyedExternalArray(LStoreKeyed* instr);
   void DoStoreKeyedFixedDoubleArray(LStoreKeyed* instr);
   void DoStoreKeyedFixedArray(LStoreKeyed* instr);
-  template <class T>
-      void PrepareKeyForKeyedOp(T* hydrogen_instr, LOperand* key);
 
   Zone* zone_;
   LPlatformChunk* const chunk_;
   MacroAssembler* const masm_;
   CompilationInfo* const info_;
 
   int current_block_;
   int current_instruction_;
   const ZoneList<LInstruction*>* instructions_;
   ZoneList<LEnvironment*> deoptimizations_;
@@ skipping 68 matching lines @@
   LCodeGen* codegen_;
   Label entry_;
   Label exit_;
   Label* external_exit_;
   int instruction_index_;
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_LITHIUM_CODEGEN_X64_H_