MIPS64: Correct register use.

TEST: mozilla/js1_5/Regress/regress-366601.
TEST: mozilla/ecma_3/ExecutionContexts/10.1.3-1

BUG=
R=paul.lind@imgtec.com

Review URL: https://codereview.chromium.org/403993002

Patch from Yu Yin <xwafish@gmail.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22489 ce2b1a6d-e550-0410-aec6-3dcde31c8c00