[dev.typeparams] cmd/compile/internal/types2: use converter functions rather than methods

This change replaces methods with functions to reduce the API surface of
types2.Type and to match the approach taken in go/types. The converter
methods for Named and TypeParam will be addressed in a follow-up CL.

Also: Fixed behavior of optype to return the underlying type for
      arguments that are not type parameters.

Change-Id: Ia369c796754bc33bbaf0c9c8478badecb729279b
Reviewed-on: https://go-review.googlesource.com/c/go/+/293291


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