Add initial hydrogenized NewStringAddStub.

src/x64/lithium-x64.cc

Index: src/x64/lithium-x64.cc
diff --git a/src/x64/lithium-x64.cc b/src/x64/lithium-x64.cc
index 281589e32cc7f42f7992369df3755281b5a542dd..eec55972ffe73da2fa956cdf7c4271bd303d4072 100644
--- a/src/x64/lithium-x64.cc
+++ b/src/x64/lithium-x64.cc
@@ -2339,8 +2339,12 @@ LInstruction* LChunkBuilder::DoStoreNamedGeneric(HStoreNamedGeneric* instr) {
 
 LInstruction* LChunkBuilder::DoStringAdd(HStringAdd* instr) {
   LOperand* context = UseFixed(instr->context(), rsi);
-  LOperand* left = UseOrConstantAtStart(instr->left());
-  LOperand* right = UseOrConstantAtStart(instr->right());
+  LOperand* left = FLAG_new_string_add
+      ? UseFixed(instr->left(), rdx)
+      : UseOrConstantAtStart(instr->left());
+  LOperand* right = FLAG_new_string_add
+      ? UseFixed(instr->right(), rax)
+      : UseOrConstantAtStart(instr->right());
   return MarkAsCall(
       DefineFixed(new(zone()) LStringAdd(context, left, right), rax), instr);
 }