Don't propagate information through phis in loop headers.
To properly do this, we'd have to iterate over CompareMaps (and their bodies) handling phis, until we have learned enough to decide which paths can be taken. For now, just disable learning from phis in loop headers.

BUG=
R=ishell@chromium.org

Review URL: https://codereview.chromium.org/147023005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19341 ce2b1a6d-e550-0410-aec6-3dcde31c8c00