MIPS: Reland "Allocation site support for monomorphic StringAdds in BinaryOps".

Port r18444 (856f92e)

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/110013005

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18447 ce2b1a6d-e550-0410-aec6-3dcde31c8c00