The validator was trying to finalize virtual register assignments in
phi cases, however, since phis may create aliases, we ended up with
an unnecessarily complex design that was the source of pretty much all
validator bugs since its introduction.

This change embraces the fact that phis may create aliases: pending
assessments (==phis) carry a bag of aliased virtual registers. 

Bug: chromium:758778
Change-Id: Ib7ded350a726fbc77e9d0ff3eeda7f00acc4de13
Reviewed-on: https://chromium-review.googlesource.com/639530
Commit-Queue: Mircea Trofin <mtrofin@chromium.org>
Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47657}