Fix Smi-based MathMinMax on x64, and reenable smi mode.

BUG=
R=jkummerow@chromium.org

Review URL: https://chromiumcodereview.appspot.com/20706002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15905 ce2b1a6d-e550-0410-aec6-3dcde31c8c00