cmd/compile: change !l.nonzero() into l.maybezero()

cmd/compile: change !l.nonzero() into l.maybezero()

  if l.maybezero()
is easier to read than
  if !l.nonzero()

Change-Id: I1183b0c0dc51fa1eed26dfc7a5a996783806a991
Reviewed-on: https://go-review.googlesource.com/c/go/+/714621
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>