Add initial hydrogenized NewStringAddStub.

src/x64/full-codegen-x64.cc

Index: src/x64/full-codegen-x64.cc
diff --git a/src/x64/full-codegen-x64.cc b/src/x64/full-codegen-x64.cc
index cc0d3d9f373dd9f100bbfa824b7fe1448066c4dd..5a343bed1eb04ddf85aa38d0fcf1293241673b4b 100644
--- a/src/x64/full-codegen-x64.cc
+++ b/src/x64/full-codegen-x64.cc
@@ -3628,11 +3628,20 @@ void FullCodeGenerator::EmitStringAdd(CallRuntime* expr) {
   ZoneList<Expression*>* args = expr->arguments();
   ASSERT_EQ(2, args->length());
 
-  VisitForStackValue(args->at(0));
-  VisitForStackValue(args->at(1));
+  if (FLAG_new_string_add) {
+    VisitForStackValue(args->at(0));
+    VisitForAccumulatorValue(args->at(1));
 
-  StringAddStub stub(STRING_ADD_CHECK_BOTH);
-  __ CallStub(&stub);
+    __ pop(rdx);
+    NewStringAddStub stub(STRING_ADD_CHECK_BOTH, NOT_TENURED);
+    __ CallStub(&stub);
+  } else {
+    VisitForStackValue(args->at(0));
+    VisitForStackValue(args->at(1));
+
+    StringAddStub stub(STRING_ADD_CHECK_BOTH);
+    __ CallStub(&stub);
+  }
   context()->Plug(rax);
 }