cmd/compile: change gc logging to report inline failure instead of success

I've been experimenting with this, success is the wrong thing to report
even though it seems to log much less.

Change-Id: I7c25a45d2f41e82b6c8dd8b0a56ba848c63fb21a
Reviewed-on: https://go-review.googlesource.com/c/go/+/223298


Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>