Support string add in crankshaft:

src/x64/lithium-codegen-x64.h

 // Copyright 2011 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 250 matching lines @@
 
   // Emits optimized code for %_IsConstructCall().
   // Caller should branch on equal condition.
   void EmitIsConstructCall(Register temp);
 
   void EmitLoadField(Register result,
                      Register object,
                      Handle<Map> type,
                      Handle<String> name);
 
-  // Emits code for pushing a constant operand.
-  void EmitPushConstantOperand(LOperand* operand);
+  // Emits code for pushing either a tagged constant, a (non-double)
+  // register, or a stack slot operand.
+  void EmitPushTaggedOperand(LOperand* operand);
 
   struct JumpTableEntry {
     explicit inline JumpTableEntry(Address entry)
         : label(),
           address(entry) { }
     Label label;
     Address address;
   };
 
   LChunk* const chunk_;
@@ skipping 69 matching lines @@
  private:
   LCodeGen* codegen_;
   Label entry_;
   Label exit_;
   Label* external_exit_;
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_LITHIUM_CODEGEN_X64_H_