Enforce the invariant that unreachable implicates an empty live set and
fix the implementation of |= and &= operator.
This is a fix-up for http://crrev.com/2694103005.

R=vegorov@chromium.org
CC=mstarzinger@chromium.org, machenbach@chromium.org
BUG=v8:5970

Review-Url: https://codereview.chromium.org/2691103008
Cr-Commit-Position: refs/heads/master@{#43251}