Improved phi reachability computation a bit.

Improved phi reachability computation a bit.

The use-def relation between phis is mainly "forwards" (i.e. from phis with
smaller IDs to ones with larger IDs), so the fixed point computation terminates
faster when iterate through the phis in a "backwards" manner. This is quite
visible in complex Mandreel-generated code, where a few hundred phis with
non-trivial use-def chains are generated.
Committed: http://code.google.com/p/v8/source/detail?r=9185

[danno, 2011-09-08 07:37:52]

LGTM