SplitBetween (formely known as Split with 3 arguments) should select split position from [start, end] instead of [start, end[. This should also improve allocation quality (remove certain redundant move patterns).

Also some minor renaming and refactoring to make register allocator code more readable.

BUG=v8:962
TEST=test/mjsunit/regress/regress-962.js

Review URL: http://codereview.chromium.org/5720001

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5969 ce2b1a6d-e550-0410-aec6-3dcde31c8c00