MIPS: Generate KeyedLoadGeneric with Hydrogen.

Port r21774 (f0462b8)

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/328173002

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21775 ce2b1a6d-e550-0410-aec6-3dcde31c8c00