Separate ConvertTaggedHoleToUndefined and CheckTaggedHole into two
separate operators, where the former is pure and just turns into
trivial control flow in the EffectControlLinearizer.

R=jarin@chromium.org

Review-Url: https://codereview.chromium.org/2236443004
Cr-Commit-Position: refs/heads/master@{#38559}