MIPS: Harmony: optimize Math.clz32.

Port r19487 (bd8c70f)

BUG=v8:2938
LOG=N
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/172533002

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19499 ce2b1a6d-e550-0410-aec6-3dcde31c8c00