go/types: avoid endless recursion in the Comparable predicate

This is a port of CL 276374 from the dev.typeparams branch. Avoid an
endless recursion in Comparable by tracking types that have already been
considered.

Fixes #43088

Change-Id: I927b29ac544df9bfb5c8c04699d57fafe6cfff73
Reviewed-on: https://go-review.googlesource.com/c/go/+/276552


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