[dev.regabi] cmd/compile: remove uses of dummy

Per https://developers.google.com/style/inclusive-documentation,
since we are editing some of this code anyway and it is easier
to put the cleanup in a separate CL.

Change-Id: Ib6b851f43f9cc0a57676564477d4ff22abb1cee5
Reviewed-on: https://go-review.googlesource.com/c/go/+/273106


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