Since both null and undefined are also marked as undetectable now, we
can just test that bit instead of having the CompareNilIC try to collect
feedback to speed up the general case (without the undetectable bit
being used).

Drive-by-fix: Update the type system to match the new handling of
undetectable in the runtime.

R=danno@chromium.org

Review URL: https://codereview.chromium.org/1722193002

Cr-Commit-Position: refs/heads/master@{#34237}