MIPS: port Dispatch on ElementsKind rather than ExternalArrayType ...

Port r8244 (c90151c)

Orig commit msg:
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.

BUG=
TEST=

Review URL: http://codereview.chromium.org//7134064
Patch from Paul Lind <plind44@gmail.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8251 ce2b1a6d-e550-0410-aec6-3dcde31c8c00