MIPS: Consolidated all the key store/load classes in the Hydrogen and Lithium space into just two: HLoadKeyed/HLoadKeyedGeneric and HStoreKeyed/HStoreKeyedGeneric LLoadKeyed/LLoadKeyedGeneric and LStoreKeyed/LStoreKeyedGeneric

Port r12839 (b8b7faa3)

BUG=
TEST=

Review URL: https://codereview.chromium.org/11369110
Patch from palfia <palfia@homejinni.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12876 ce2b1a6d-e550-0410-aec6-3dcde31c8c00