MIPS: Minor fixes and additions needed for Turbofan.

These small changes can be landed indendently to avoid
cluttering up the initial turbofan CL.

BUG=
R=balazs.kilvady@imgtec.com

Review URL: https://codereview.chromium.org/602603005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24193 ce2b1a6d-e550-0410-aec6-3dcde31c8c00