Fix smi-based math floor.

BUG=chromium:270268
R=svenpanne@chromium.org

Review URL: https://chromiumcodereview.appspot.com/22623007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16128 ce2b1a6d-e550-0410-aec6-3dcde31c8c00