MIPS: Make KeyedLoads from a sloppy arguments array use a handler.

Port r24120 (eed5963)

Original commit message:
Before, a custom stub was installed.

BUG=
R=paul.lind@imgtec.com

Review URL: https://codereview.chromium.org/594563003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24126 ce2b1a6d-e550-0410-aec6-3dcde31c8c00