cmd/compile/internal/types2: break cycles in invalid types
This CL reverses the change in CL 284254 (which was ported
to types2) which originated in CL 240901 to address a crash
in a test created by a fuzzer (that crash appears to be
avoided in other ways, now).
This exposed another bug in typeset.go where we don't look
for the underlying type when testing if a type is an interface
or not. Fixed that as well.
Adjusted a test case that now doesn't report an error anymore
(which is good).
Fixes #48819.
Change-Id: I611d68e053d6b8a2f7176d0cd5a44da2df28ad21
Reviewed-on: https://go-review.googlesource.com/c/go/+/354329
Trust: Robert Griesemer <gri@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>