The main check here is that liveness now crashes if it finds an instruction
using a variable that should be tracked but is not.

Comments and adjustments in nodarg to explain what's going on and
to remove the "-1" argument added a few months ago, plus a sketch
of a future simplification.

The need for n.Orig in the earlier CL seems to have been an intermediate
problem rather than fundamental: the new explanations in nodarg make
clear that nodarg is not causing the problem I thought, and in fact now
using n instead of n.Orig works fine in plive.go.

Change-Id: I3f5cf9f6e4438a6d27abac7d490e7521545cd552
Reviewed-on: https://go-review.googlesource.com/23450
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>