Support string add in crankshaft:

src/ia32/lithium-ia32.h

Index: src/ia32/lithium-ia32.h
diff --git a/src/ia32/lithium-ia32.h b/src/ia32/lithium-ia32.h
index 9ace8f869b68c20cf59ccbd8d7880fad85584d91..d5d6ae539be8385c1c53e940530c83a536c67435 100644
--- a/src/ia32/lithium-ia32.h
+++ b/src/ia32/lithium-ia32.h
@@ -39,6 +39,7 @@ namespace internal {
 // Forward declarations.
 class LCodeGen;
 
+
 #define LITHIUM_ALL_INSTRUCTION_LIST(V)         \
   V(ControlInstruction)                         \
   V(Call)                                       \
@@ -154,6 +155,7 @@ class LCodeGen;
   V(StoreKeyedSpecializedArrayElement)          \
   V(StoreNamedField)                            \
   V(StoreNamedGeneric)                          \
+  V(StringAdd)                                  \
   V(StringCharCodeAt)                           \
   V(StringCharFromCode)                         \
   V(StringLength)                               \
@@ -1769,6 +1771,21 @@ class LStoreKeyedGeneric: public LTemplateInstruction<0, 4, 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) {