MIPS: Improve compare and branch combining.

TEST=
BUG=
R=paul.lind@imgtec.com

Review URL: https://codereview.chromium.org/700873002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25120 ce2b1a6d-e550-0410-aec6-3dcde31c8c00