Before, we disabled code motion for check instructions on the final
optimization attempt.  It is unsafe to prevent movement of check
instructions but to allow movement of instructions that assume the checks
were performed.

Review URL: http://codereview.chromium.org/6378012

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6485 ce2b1a6d-e550-0410-aec6-3dcde31c8c00