MIPS: Save one branch for normal heap number un-tagging.

Port r16856 (edeb2d3)

BUG=
R=gergely@homejinni.com

Review URL: https://codereview.chromium.org/24307005

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

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16871 ce2b1a6d-e550-0410-aec6-3dcde31c8c00