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 51e3220c9fe897db10e0cea96771862ade0574b7..c5ca6e093098cf0f3b3f78fbc65edcbfd4b18043 100644
--- a/src/x64/lithium-x64.cc
+++ b/src/x64/lithium-x64.cc
@@ -2307,8 +2307,12 @@ LInstruction* LChunkBuilder::DoStoreNamedGeneric(HStoreNamedGeneric* instr) {
 
 
 LInstruction* LChunkBuilder::DoStringAdd(HStringAdd* instr) {
-  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(left, right), rax),
                     instr);
 }