Merged r18030 into 3.22 branch.

MIPS: Fix and simplify code aging.

R=ulan@chromium.org
BUG=

Review URL: https://codereview.chromium.org/136683003

git-svn-id: https://v8.googlecode.com/svn/branches/3.22@18568 ce2b1a6d-e550-0410-aec6-3dcde31c8c00