MIPS: Fix register usage in softfloat code path.

BUG=none
TEST=mjsunit/math-floor-part2.js

Review URL: https://codereview.chromium.org/12413032
Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14099 ce2b1a6d-e550-0410-aec6-3dcde31c8c00