[dev.regabi] cmd/compile: prepare for package ir

The next CL will introduce a package ir to hold the IR definitions.
This CL adjusts a few names and makes a few other minor changes
to make the next CL - an automated one - smoother.

Change-Id: Ie787a34732efd5b3d171bf0c1220b6dd91994ce3
Reviewed-on: https://go-review.googlesource.com/c/go/+/272251


Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>