cmd/compile/internal/types2: fix method lookup for type-parameter based types

Pointers to type parameters don't have methods, but pointers to
defined types whose underlying types are type parameters may have
methods. Fix the respective test.

For #47747.

Change-Id: I1de47be094ed9297f0e7782538011657c37c5adc
Reviewed-on: https://go-review.googlesource.com/c/go/+/342990


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>