cmd/compile: make nilcheck more careful about statement relocations

The earlier code was picking nodes that were "poor choices" and
thus sometimes losing statements altogether.

Change-Id: Ibe5ed800ffbd3c926c0ab1bc10c77d72d3042e45
Reviewed-on: https://go-review.googlesource.com/c/go/+/198478


Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>