cmd/link: add debug print in deadcode

This matches the prints that deadcode prints later
as the algorithm progresses under -v=2.
It helps to see the initial conditions with -v=2 as well.

Change-Id: I06ae86fe9bd8314d003148f3d941832c9b10aef1
Reviewed-on: https://go-review.googlesource.com/c/go/+/288817


Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>