This way we avoid the %_IsSmi magic that is required in TurboFan to
(efficiently) check abitrary context slots for smi 0. Checking against
"the hole" is common in the AstGraphBuilder and "the hole" is also used
to mark other context slots as not initialized.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/1484723003

Cr-Commit-Position: refs/heads/master@{#32407}