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