port 9e644881 (r32407)

  original commit message:
  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.

BUG=

Review URL: https://codereview.chromium.org/1486913002

Cr-Commit-Position: refs/heads/master@{#32441}