Dispatch on ElementsKind rather than ExternalArrayType when generating ICs and Cranks...

Dispatch on ElementsKind rather than ExternalArrayType when generating ICs and Crankshaft code for many element operations. This is preparation to be able to share more code in the various element accessor implementations.

Merge logic to maintain external array and fast element stub caches.

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/7112010

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8244 ce2b1a6d-e550-0410-aec6-3dcde31c8c00