MIPS: Fix DCHECK failure in simulator for fp32 mode.

TEST=mjsunit tests in debug
BUG=
R=paul.lind@imgtec.com

Review URL: https://codereview.chromium.org/467203002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23109 ce2b1a6d-e550-0410-aec6-3dcde31c8c00