cmd/compile: print lines missing is_stmt if testing.Verbose()

helpful for debugging failures to figure out which lines
disappeared to cause the failure.

Change-Id: Id42b64a65f76eed47e01003f05346bc09cec27e8
Reviewed-on: https://go-review.googlesource.com/c/go/+/188019


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