MIPS: Allow comparison in UINT32 mode.

Port r21355 (932c5de)

Original commit message:
Shamelessly based on parts of https://codereview.chromium.org/288853003/. :-)

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/296453003

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21369 ce2b1a6d-e550-0410-aec6-3dcde31c8c00