MIPS: Add MEGAMORPHIC state support for KeyedStoreIC.

Port r24500 (e10f342)

BUG=
R=paul.lind@imgtec.com

Review URL: https://codereview.chromium.org/645593006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24503 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Merge remote-tracking branch 'upstream/master'