Support string add in crankshaft:

src/x64/lithium-x64.cc

Index: src/x64/lithium-x64.cc
diff --git a/src/x64/lithium-x64.cc b/src/x64/lithium-x64.cc
index 62573ca46ff5aefb055557319325b3506bae018e..ddc227df7b1700f4d64d49f627b7fe9c704462e6 100644
--- a/src/x64/lithium-x64.cc
+++ b/src/x64/lithium-x64.cc
@@ -1940,6 +1940,13 @@ LInstruction* LChunkBuilder::DoStoreNamedGeneric(HStoreNamedGeneric* instr) {
 }
 
 
+LInstruction* LChunkBuilder::DoStringAdd(HStringAdd* instr) {
+  LOperand* left = UseOrConstantAtStart(instr->left());
+  LOperand* right = UseOrConstantAtStart(instr->right());
+  return MarkAsCall(DefineFixed(new LStringAdd(left, right), rax), instr);
+}
+
+
 LInstruction* LChunkBuilder::DoStringCharCodeAt(HStringCharCodeAt* instr) {
   LOperand* string = UseRegister(instr->string());
   LOperand* index = UseRegisterOrConstant(instr->index());