MIPS: Fix uint32-to-smi conversion in Lithium.

Port r17441 (f1968f4)

BUG=chromium:309623
TEST=mjsunit/regress/regress-crbug-309623
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/49783010

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17451 ce2b1a6d-e550-0410-aec6-3dcde31c8c00