MIPS: Add initial hydrogenized NewStringAddStub.

src/mips/code-stubs-mips.cc

Index: src/mips/code-stubs-mips.cc
diff --git a/src/mips/code-stubs-mips.cc b/src/mips/code-stubs-mips.cc
index a0e990ccb06350a36ceafca6b2715252f2c15388..f053c7942cb4acef733139aac2f8246ee9b54360 100644
--- a/src/mips/code-stubs-mips.cc
+++ b/src/mips/code-stubs-mips.cc
@@ -291,6 +291,17 @@ void ElementsTransitionAndStoreStub::InitializeInterfaceDescriptor(
 }
 
 
+void NewStringAddStub::InitializeInterfaceDescriptor(
+    Isolate* isolate,
+    CodeStubInterfaceDescriptor* descriptor) {
+  static Register registers[] = { a1, a0 };
+  descriptor->register_param_count_ = 2;
+  descriptor->register_params_ = registers;
+  descriptor->deoptimization_handler_ =
+      Runtime::FunctionForId(Runtime::kStringAdd)->entry;
+}
+
+
 #define __ ACCESS_MASM(masm)