Merged r19500 into 3.23 branch.

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

R=jkummerow@chromium.org
BUG=

Review URL: https://codereview.chromium.org/173703003

git-svn-id: https://v8.googlecode.com/svn/branches/3.23@19508 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Merge remote-tracking branch 'upstream/3.23' into 3.23