[dev.regabi] cmd/compile: separate various from Main

Move various code out of Main itself and into helper functions
that can be moved into other packages as package gc splits up.

Similarly, move order and instrument inside walk to reduce the amount
of API surface needed from the eventual package walk.

Change-Id: I7849258038c6e39625a0385af9c0edd6a3b654a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/279304


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