Costs an extra move because the result is calculated in a scratch register
and only moved to destination on non-overflow in order to preserve source
registers in case of overflow.

Review URL: http://codereview.chromium.org/2823004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4877 ce2b1a6d-e550-0410-aec6-3dcde31c8c00