[dev.regabi] cmd/compile: add custom statement Node implementations

[dev.regabi] cmd/compile: add custom statement Node implementations

These are fairly rote implementations of structs appropriate to
each Op (or group of Ops).

The names of these are unknown except to ir.NodAt for now.
A later, automated change will introduce direct use of the types
throughout package gc.

Passes buildall w/ toolstash -cmp.

Change-Id: Ie9835fcd2b214fda5b2149e187af369d76534487
Reviewed-on: https://go-review.googlesource.com/c/go/+/274108
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>