[dev.typeparams] cmd/compile/internal/types2: cleanups around receiver type checks

[dev.typeparams] cmd/compile/internal/types2: cleanups around receiver type checks

Generic receiver types may be defined such that an instantiated
receiver ends up being a pointer type. Disallow them as we do
for non-generic receivers.

Change-Id: I6612a52615a2999375c35aa1d69ab42f37d9f55d
Reviewed-on: https://go-review.googlesource.com/c/go/+/333770
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>