MIPS: Eliminate map checks of constant values.

Port r15819 (a453a0e)

BUG=

Review URL: https://codereview.chromium.org/19876006
Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15831 ce2b1a6d-e550-0410-aec6-3dcde31c8c00