MIPS: Refine CountOperation of FullCodeGen.

Port r17547 (a4b59e4)

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/64893002

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17572 ce2b1a6d-e550-0410-aec6-3dcde31c8c00