[dev.regabi] cmd/compile: implement copy for nodes

[dev.regabi] cmd/compile: implement copy for nodes

Put each node in charge of making copies of its own slices.
This removes a generic use of Body, SetBody, and so on
in func Copy, heading toward removing those even from
being used in package ir.

Passes buildall w/ toolstash -cmp.

Change-Id: I249b7fe54cf72e9d2f0467b10f3f257abf9b29b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/275374
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>