[dev.regabi] cmd/compile: setup for new Node implementations

[dev.regabi] cmd/compile: setup for new Node implementations

Start a list of which ops are valid for the default
node struct implementation (currently all of them).

Add a Node implementation helper for a minimal node.

Passes buildall w/ toolstash -cmp.

Change-Id: I7ae45f2cf2be85013cb71ab00524be53f243e13d
Reviewed-on: https://go-review.googlesource.com/c/go/+/274088
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>