Ancestor comparison was the wrong way around, effectively
disabling the def-must-dominate-use check.

Update #15084

Change-Id: Ic56d674c5000569d2cc855bbb000a60eae517c7c
Reviewed-on: https://go-review.googlesource.com/22330
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>