Support string add in crankshaft:

src/x64/lithium-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 134 matching lines @@
   V(SmiUntag)                                   \
   V(StackCheck)                                 \
   V(StoreContextSlot)                           \
   V(StoreGlobalCell)                            \
   V(StoreGlobalGeneric)                         \
   V(StoreKeyedFastElement)                      \
   V(StoreKeyedGeneric)                          \
   V(StoreKeyedSpecializedArrayElement)          \
   V(StoreNamedField)                            \
   V(StoreNamedGeneric)                          \
+  V(StringAdd)                                  \
   V(StringCharCodeAt)                           \
   V(StringCharFromCode)                         \
   V(StringLength)                               \
   V(SubI)                                       \
   V(TaggedToI)                                  \
   V(ToFastProperties)                           \
   V(Throw)                                      \
   V(Typeof)                                     \
   V(TypeofIs)                                   \
   V(TypeofIsAndBranch)                          \
@@ skipping 1512 matching lines @@
 
   virtual void PrintDataTo(StringStream* stream);
 
   LOperand* object() { return inputs_[0]; }
   LOperand* key() { return inputs_[1]; }
   LOperand* value() { return inputs_[2]; }
   bool strict_mode() { return hydrogen()->strict_mode(); }
 };
 
 
+class LStringAdd: public LTemplateInstruction<1, 2, 0> {
+ public:
+  LStringAdd(LOperand* left, LOperand* right) {
+    inputs_[0] = left;
+    inputs_[1] = right;
+  }
+
+  DECLARE_CONCRETE_INSTRUCTION(StringAdd, "string-add")
+  DECLARE_HYDROGEN_ACCESSOR(StringAdd)
+
+  LOperand* left() { return inputs_[0]; }
+  LOperand* right() { return inputs_[1]; }
+};
+
+
 class LStringCharCodeAt: public LTemplateInstruction<1, 2, 0> {
  public:
   LStringCharCodeAt(LOperand* string, LOperand* index) {
     inputs_[0] = string;
     inputs_[1] = index;
   }
 
   DECLARE_CONCRETE_INSTRUCTION(StringCharCodeAt, "string-char-code-at")
   DECLARE_HYDROGEN_ACCESSOR(StringCharCodeAt)
 
@@ skipping 459 matching lines @@
   DISALLOW_COPY_AND_ASSIGN(LChunkBuilder);
 };
 
 #undef DECLARE_HYDROGEN_ACCESSOR
 #undef DECLARE_INSTRUCTION
 #undef DECLARE_CONCRETE_INSTRUCTION
 
 } }  // namespace v8::int
 
 #endif  // V8_X64_LITHIUM_X64_H_