MIPS: Fix test262 regression.

This regression was introduced by r13705 (fd03c7d6)

BUG=test262/data/test/suite/ch11/11.13/11.13.2

Review URL: https://codereview.chromium.org/12844005
Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13944 ce2b1a6d-e550-0410-aec6-3dcde31c8c00