Convert KeyedLoad indexed interceptor case to a Handler.

Currently, KeyedLoads on objects with indexed interceptors are handled with a
special stub. Instead, key on the map and handler mechanism for more uniform
treatment.

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/575373004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24042 ce2b1a6d-e550-0410-aec6-3dcde31c8c00