This includes fixes for the overflow case spotted in mjsunit/mul-exhaustive-part4.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/212573002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@20267 ce2b1a6d-e550-0410-aec6-3dcde31c8c00