cmd/compile/internal/types2: clarify is/underIs semantics and implementation

The behavior of is/underIs was murky with the presence of a top type term
(corresponding to a type set that is not constrained by any types, yet the
function argument f of is/underIs was called with that term).

Change is/underIs to call f explicitly for existing specific type terms,
otherwise return the result of f(nil). Review all uses of is/underIs and
variants.

This makes the conversion code slightly more complicated because we need
to explicitly exclude type parameters without specific types; but the
code is clearer now.

Change-Id: I6115cb46f7f2a8d0f54799aafff9a67c4cca5e30
Reviewed-on: https://go-review.googlesource.com/c/go/+/358594


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>