Fix overflow check computation for Smi Phis

BUG=v8:2836
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/22629011

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16180 ce2b1a6d-e550-0410-aec6-3dcde31c8c00