Support string add in crankshaft:

src/x64/lithium-x64.h

Index: src/x64/lithium-x64.h
diff --git a/src/x64/lithium-x64.h b/src/x64/lithium-x64.h
index 15bb8941c328edb3bd94925c65f6a58013c38ac7..b532236c27b20da0d996871e57e31113a3b105c8 100644
--- a/src/x64/lithium-x64.h
+++ b/src/x64/lithium-x64.h
@@ -152,6 +152,7 @@ class LCodeGen;
   V(StoreKeyedSpecializedArrayElement)          \
   V(StoreNamedField)                            \
   V(StoreNamedGeneric)                          \
+  V(StringAdd)                                  \
   V(StringCharCodeAt)                           \
   V(StringCharFromCode)                         \
   V(StringLength)                               \
@@ -1684,6 +1685,21 @@ class LStoreKeyedGeneric: public LTemplateInstruction<0, 3, 0> {
 };
 
 
+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) {