MIPS: Merge KeyedLoad and NamedLoad stub compiler code.

Port r13579 (e640269)

BUG=

Review URL: https://chromiumcodereview.appspot.com/12186003
Patch from Paul Lind <plind44@gmail.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13580 ce2b1a6d-e550-0410-aec6-3dcde31c8c00