[dev.regabi] cmd/compile: generalize ir/mknode.go

This CL generalizes ir/mknode.go to get rid of most of almost all of
its special cases for node field types. The only remaining speciale
case now is Field, which doesn't implement Node any more, but perhaps
should.

To help with removing special cases, node fields can now be tagged
with `mknode:"-"` so that mknode ignores them when generating its
helper methods. Further, to simplify skipping all of the orig fields,
a new origNode helper type is added which declares an orig field
marked as `mknode:"-"` and also provides the Orig and SetOrig methods
needed to implement the OrigNode interface.

Passes toolstash -cmp.

Change-Id: Ic68d4f0a9d2ef6e57e9fe87cdc641e5c4859830b
Reviewed-on: https://go-review.googlesource.com/c/go/+/280674


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>