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 632b065168e2f75cb355b0756fef46dd16283e28..2b1a0259d0e4532ee31a6b727799846022a80e13 100644
--- a/src/ia32/lithium-ia32.cc
+++ b/src/ia32/lithium-ia32.cc
@@ -2485,8 +2485,12 @@ 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);
 }