Change CompareNilIC to return true/false instead of 1/0, and use the
CompareNilICStub in TurboFan for abstract equality with null/undefined.
This way we don't always run into the %Equals runtime fallback when
comparing objects with null/undefined.

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/1417503008

Cr-Commit-Position: refs/heads/master@{#31596}