MIPS: Move register conventions out of the IC classes.

Port r23391 (c3bf1dc)

Original commit message:
A change to a convention shouldn't require recompilation of ic.h/.cc.

BUG=
R=paul.lind@imgtec.com, vogelheim@chromium.org

Review URL: https://codereview.chromium.org/508673002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23420 ce2b1a6d-e550-0410-aec6-3dcde31c8c00