go/types, types2: slightly reorganize method receiver checking

go/types, types2: slightly reorganize method receiver checking

- move receiver checks up, closer to where the receiver is collected
- adjust some comments after verifying against some test cases
- removed some minor discrepancies between the two type checkers

For #51343.

Change-Id: I75b58efbed1e408df89b8d6536e6c6da45740f93
Reviewed-on: https://go-review.googlesource.com/c/go/+/593336
Auto-Submit: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>