MIPS: Fix performance regression on sunspider string-unpack-code.

Regression surfaced with r19009, but latent error was present since
r17767.

BUG=
R=dusan.milosavljevic@rt-rk.com

Review URL: https://codereview.chromium.org/171793005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19500 ce2b1a6d-e550-0410-aec6-3dcde31c8c00