[dev.typeparams] cmd/compile/internal/types2: use same sort criteria for methods as compiler

Note: This invalidates the implementation of MethodSet further (it
also has not been updated to accomodate for type parameters). But
types2 doesn't make use of it. We should remove it.

Change-Id: Ia2601bdd59b3f3ee0b72bc2512153c42bf5053b5
Reviewed-on: https://go-review.googlesource.com/c/go/+/285994


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>