MIPS: Let the register allocator handle the context register.

Port r16993 (afba553d)

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/25420002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17059 ce2b1a6d-e550-0410-aec6-3dcde31c8c00