Better support for 'polymorphic' JS and external arrays

Allow  keyed store/load stubs to switch between external array and fast JS arrays without forcing a state transition to the generic stub.

There CL consists of two pieces of functionality. First, code stubs for fast element arrays don't immediately transition to the MEGAMORPHIC state when there's a map mismatch. Second, two ICs are cached per map for fast elements, the MONOMORPHIC version, and a new MEGAMORPHIC version that handles two or more different maps and dispatches to shared stubs to perform the array operation.

Review URL: http://codereview.chromium.org/7036016

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7935 ce2b1a6d-e550-0410-aec6-3dcde31c8c00