Extend sminess propagator to process branch instructions.

Extend sminess propagator to process branch instructions.

When doing comparisons like (x == null) and the comparison
is dominated by a smi-check, we can fold the comparison away.

One example where this occurs is the List constructor where the length
parameter is checked for null. If not null, the length of the list
is fixed and known and can be used to eliminate bounds checks.
Committed: https://code.google.com/p/dart/source/detail?r=16037

[Vyacheslav Egorov (Google), 2012-12-12 13:39:21]

lgtm

but we should really work towards a unified type propagation.