MIPS: Fix r22021 “The IC exposes a register definition.” after r22018.

BUG=
R=palfia@homejinni.com

Review URL: https://codereview.chromium.org/344543004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22024 ce2b1a6d-e550-0410-aec6-3dcde31c8c00