go/types, types2: introduce `isValid` predicate and use throughout

go/types, types2: introduce `isValid` predicate and use throughout

Preparation for Alias type nodes. Using a predicate will ensure
that alias indirection can be taken care of when needed, eventually.

Change-Id: I689fab6052060eb6bcb2eeac28ba09fdb73f6868
Reviewed-on: https://go-review.googlesource.com/c/go/+/528695
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>