Don't propagate information through phis in loop headers.

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

Committed: https://code.google.com/p/v8/source/detail?r=19341

[Toon Verwaest, 2014-02-12 18:28:58]

PTAL

[Igor Sheludko, 2014-02-12 18:30:17]

lgtm

[Toon Verwaest, 2014-02-12 18:30:50]

Committed patchset #2 manually as r19341 (presubmit successful).