MIPS: Inline number to string conversion for string addition into BinaryOp(Stub).

Port r17290 (b5fabb92)

Original commit message:
This fixes a performance regression that was caused by converting the
BinaryOpStub to a Hydrogen code stub. It also fixes a leftover TODO wrt.
the handling of Number*String or String*Number versions of the stub.

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/32473007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@17304 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/mips/code-stubs-mips.cc

@@ -66,7 +66,8 @@ void NumberToStringStub::InitializeInterfaceDescriptor(
   static Register registers[] = { a0 };
   descriptor->register_param_count_ = 1;
   descriptor->register_params_ = registers;
-  descriptor->deoptimization_handler_ = NULL;
+  descriptor->deoptimization_handler_ =
+      Runtime::FunctionForId(Runtime::kNumberToString)->entry;
 }