Add initial hydrogenized NewStringAddStub.

src/ia32/full-codegen-ia32.cc

Index: src/ia32/full-codegen-ia32.cc
diff --git a/src/ia32/full-codegen-ia32.cc b/src/ia32/full-codegen-ia32.cc
index b7ddeac0e428d2846aec3e5573cf90ac7a3ff5d2..f3ac9c5cbafebc73ed2d57a583930d749559fcb2 100644
--- a/src/ia32/full-codegen-ia32.cc
+++ b/src/ia32/full-codegen-ia32.cc
@@ -3671,11 +3671,20 @@ 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(edx);
+    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(eax);
 }