Add initial hydrogenized NewStringAddStub.

src/arm/lithium-arm.cc

Index: src/arm/lithium-arm.cc
diff --git a/src/arm/lithium-arm.cc b/src/arm/lithium-arm.cc
index 5fb0e533b86848dafd045a68944039e0fca2eb0b..6d02ba1b1bddb1f470d9eed0275ab7b0b94496b6 100644
--- a/src/arm/lithium-arm.cc
+++ b/src/arm/lithium-arm.cc
@@ -2413,8 +2413,12 @@ LInstruction* LChunkBuilder::DoStoreNamedGeneric(HStoreNamedGeneric* instr) {
 
 LInstruction* LChunkBuilder::DoStringAdd(HStringAdd* instr) {
   LOperand* context = UseFixed(instr->context(), cp);
-  LOperand* left = UseRegisterAtStart(instr->left());
-  LOperand* right = UseRegisterAtStart(instr->right());
+  LOperand* left = FLAG_new_string_add
+      ? UseFixed(instr->left(), r1)
+      : UseRegisterAtStart(instr->left());
+  LOperand* right = FLAG_new_string_add
+      ? UseFixed(instr->right(), r0)
+      : UseRegisterAtStart(instr->right());
   return MarkAsCall(
       DefineFixed(new(zone()) LStringAdd(context, left, right), r0),
       instr);