cmd/compile: move nodarg to walk.go

cmd/compile: move nodarg to walk.go

Its sole use is in walk.go. 100% code movement.

gsubr.go increasingly contains backend-y things.
With a few more relocations, it could probably be
fruitfully renamed progs.go.

Change-Id: I61ec5c2bc1f8cfdda64c6d6f580952c154ff60e0
Reviewed-on: https://go-review.googlesource.com/41972
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>