[dev.typeparams] cmd/compile/internal/types2: clean up type set/union intersection

- Eliminate the need for bottom type: This is now represented by
  an empty union (denoting the set of no types).

- Clean up type set intersection and incorporate tilde information
  in intersection operation and satisfaction tests.

- Minor cleanups along the way.

- Note: The intersection algorithm does not always compute the
        largest possible intersection. To be addressed in a follow-up CL.

Change-Id: I7fa19df5996da36a4d8f29300d30a0aa4d8a3e5c
Reviewed-on: https://go-review.googlesource.com/c/go/+/323354


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>