cmd/compile: attempt to preserve statements when prove removes code

This was a cause of some statements being lost.

Change-Id: I81c95dcf3df6ed8a03b7578a27f9b21d33b3cf39
Reviewed-on: https://go-review.googlesource.com/c/go/+/198484


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