[dev.regabi] cmd/compile: separate dowidth better

[dev.regabi] cmd/compile: separate dowidth better

Having a global MaxWidth lets us avoid needing to
refer to thearch from split-out packages when all
they need is thearch.MAXWIDTH.

And make a couple interface changes to let dowidth
avoid importing package ir directly.
Then it can move into package types.

Change-Id: I2c12e8e22252597530e648848320e19bdd490a01
Reviewed-on: https://go-review.googlesource.com/c/go/+/279302
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>