MIPS: Eliminate special keyed load string stub in favor of uniform handlers.

Port r24661 (5499efb)

Original commit message:
KeyedLoadIC installs a special case if the receiver is a string.
Although there are several maps for strings, in practice we seem to
be able to treat them individually because a given KeyedLoad site
only sees 1-2 string types.

BUG=
R=paul.lind@imgtec.com

Review URL: https://codereview.chromium.org/657413002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24668 ce2b1a6d-e550-0410-aec6-3dcde31c8c00