[dev.regabi] cmd/compile: stop using ONONAME with Name

This CL changes NewDeclNameAt to take an Op argument to set the Op up
front, and updates all callers to provide the appropriate Op. This
allows dropping the Name.SetOp method.

Passes buildall w/ toolstash -cmp.

Change-Id: I20e580f62d3c8a81223d1c162327c11b37bbf3f0
Reviewed-on: https://go-review.googlesource.com/c/go/+/279314


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