MIPS: Refactoring and cleanup of control instructions.

Port r15513 (6f50861)

BUG=

Review URL: https://codereview.chromium.org/18359004
Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15527 ce2b1a6d-e550-0410-aec6-3dcde31c8c00