cmd/9g: use nopout in excise

cmd/9g: use nopout in excise

In addition to duplicating the logic, the old code was
clearing the line number, which led to missing source line
information in the -S output.

Also fix nopout, which was incomplete.

Change-Id: Ic2b596a2f9ec2fe85642ebe125cca8ef38c83085
Reviewed-on: https://go-review.googlesource.com/3512
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Dave Cheney <dave@cheney.net>