MIPS: Inline Math.fround in optimized code.

Port r22665 (7e3d70d)

BUG=v8:3469
LOG=N
R=paul.lind@imgtec.com

Review URL: https://codereview.chromium.org/425053002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22682 ce2b1a6d-e550-0410-aec6-3dcde31c8c00