Also properly support loading from holey double element backing stores
in JSNativeContextSpecialization. This adds a new simplified operator
NumberIsHoleNaN, which checks whether a certain value is the special NaN
that we use to encode "the hole" in holey double element backing stores.

R=jarin@chromium.org
BUG=v8:4470
LOG=n

Review URL: https://codereview.chromium.org/1448343002

Cr-Commit-Position: refs/heads/master@{#32039}