... such that we have only a single representation for special
constants such as undefined, namely the corresponding bitset.
With this CL the following property holds:
    t1.IsSingleton() /\ t2.Is(t1) => t1.Is(t2)

Also clean up the Type interface and improve test coverage a little.

Change-Id: I074e20047c92e2c8215c2d438f2627f4ffdbc409
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2096631
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Auto-Submit: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66684}