Add initial hydrogenized NewStringAddStub.

src/ia32/lithium-ia32.cc

Index: src/ia32/lithium-ia32.cc
diff --git a/src/ia32/lithium-ia32.cc b/src/ia32/lithium-ia32.cc
index 80dbdd5cda7610ebceb258c08927a0fc88f92d08..c0818be064082bfebe0016b2978398bc1ada29e6 100644
--- a/src/ia32/lithium-ia32.cc
+++ b/src/ia32/lithium-ia32.cc
@@ -2486,8 +2486,12 @@ LInstruction* LChunkBuilder::DoStoreNamedGeneric(HStoreNamedGeneric* instr) {
 
 LInstruction* LChunkBuilder::DoStringAdd(HStringAdd* instr) {
   LOperand* context = UseFixed(instr->context(), esi);
-  LOperand* left = UseOrConstantAtStart(instr->left());
-  LOperand* right = UseOrConstantAtStart(instr->right());
+  LOperand* left = FLAG_new_string_add
+      ? UseFixed(instr->left(), edx)
+      : UseOrConstantAtStart(instr->left());
+  LOperand* right = FLAG_new_string_add
+      ? UseFixed(instr->right(), eax)
+      : UseOrConstantAtStart(instr->right());
   LStringAdd* string_add = new(zone()) LStringAdd(context, left, right);
   return MarkAsCall(DefineFixed(string_add, eax), instr);
 }