MIPS: Add initial hydrogenized NewStringAddStub.

src/mips/lithium-mips.cc

Index: src/mips/lithium-mips.cc
diff --git a/src/mips/lithium-mips.cc b/src/mips/lithium-mips.cc
index edb1206b01e84ff1148d1cc66970d8a5b383ae86..943d9476f43daca9a890f16371662c5be897a591 100644
--- a/src/mips/lithium-mips.cc
+++ b/src/mips/lithium-mips.cc
@@ -2335,8 +2335,12 @@ 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(), a1)
+      : UseRegisterAtStart(instr->left());
+  LOperand* right = FLAG_new_string_add
+      ? UseFixed(instr->right(), a0)
+      : UseRegisterAtStart(instr->right());
   return MarkAsCall(
       DefineFixed(new(zone()) LStringAdd(context, left, right), v0),
       instr);