MIPS: First step to cleanup the power-of-2 mess.

Port r23617 (1ad2f2a)

TEST=base-unittests,cctest,mjsunit
BUG=
R=paul.lind@imgtec.com

Review URL: https://codereview.chromium.org/519283005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23626 ce2b1a6d-e550-0410-aec6-3dcde31c8c00