cmd/compile: fix case where func-valued field of a generic type is called

cmd/compile: fix case where func-valued field of a generic type is called

Added test example orderedmap.go (binary search tree) that requires this
fix (calling function compare in _Map).

Also added new tests slices.go and metrics.go that just work.

Change-Id: Ifa5f42ab6eee9aa54c40f0eca19e00a87f8f608a
Reviewed-on: https://go-review.googlesource.com/c/go/+/301829
Trust: Dan Scales <danscales@google.com>
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>