This unconditionally enables zapping of old optimized code maps and
unifies the various zapping paths. The unconditional zapping, even if
heap verification is off, is needed because slots in the code map have
not been recorded and evacuation invariants break.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/1410833009

Cr-Commit-Position: refs/heads/master@{#31895}