Fix MathFloorOfDiv canonicalization ASSERT failures

- remove outdated ASSERT about instruction type
- add inserted HChanges to the graph

R=danno@chromium.org

Review URL: https://codereview.chromium.org/17094005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15166 ce2b1a6d-e550-0410-aec6-3dcde31c8c00