Tweaked FlooringDiv back to TruncatingDiv again.

This is a first step towards getting the flooring division by a
constant working again (which will be a separate CL).

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/197873029

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20048 ce2b1a6d-e550-0410-aec6-3dcde31c8c00