MIPS: Reland 21774: Generate KeyedLoadGeneric with Hydrogen

Port r21781 (4869c528)

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/327373002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21784 ce2b1a6d-e550-0410-aec6-3dcde31c8c00