[dev.typeparams] cmd/compile/internal/types2: factor out sorting of methods

Cleanup and first step towards uniformly changing the sort criteria.

Change-Id: I0a7b6a10b5b646fc83f4897e4915ef4dae24aa66
Reviewed-on: https://go-review.googlesource.com/c/go/+/285993


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