[dev.regabi] cmd/compile: add Node field getters and setters

The goal is to move Node to being an interface and then break
up the one big struct into many implementations.

Step 1 is to convert all current uses of Node to only use methods,
so that the existing algorithms keep working even as the underlying
implementations are adjusted.

Step 0 - this CL - is to add the getters and setters for Step 1.

Change-Id: I0570d8727c3ccb64113627bb9bebcb0dc39da07a
Reviewed-on: https://go-review.googlesource.com/c/go/+/273007


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>