cmd/compile/loopvar: adjust logging messages

cmd/compile/loopvar: adjust logging messages

Michael Stapelberg thought the former messages had
upside potential, Russ and I agreed.

Also slightly tweaked the json logging, not sure if
anyone will use it but it should at least be okay.

Change-Id: Iaab75114dd5f5d8f011fab22d32b57abc0272815
Reviewed-on: https://go-review.googlesource.com/c/go/+/493135
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: David Chase <drchase@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Michael Stapelberg <stapelberg@google.com>