go/types, types2: factor out method receiver validation
While at it, slightly regroup surounding code for clarity.
For #51343.
Change-Id: Ibb3a58c2ea138afae0be5315d98f698d7633b22a
Reviewed-on: https://go-review.googlesource.com/c/go/+/593455 Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com> Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>