MIPS: Simplify known successor block lowering.

Port r21589 (a9f3228)

BUG=
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/306053011

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21610 ce2b1a6d-e550-0410-aec6-3dcde31c8c00