[dev.regabi] cmd/compile: rewrite code to use DeclaredBy

[dev.regabi] cmd/compile: rewrite code to use DeclaredBy

Passes buildall w/ toolstash -cmp.

Updates #42990.

[git-generate]
cd src/cmd/compile/internal/gc
rf '
ex {
  import "cmd/compile/internal/ir"
  var x, stmt ir.Node
  x.Name() != nil && x.Name().Defn == stmt ->  ir.DeclaredBy(x, stmt)
  x.Name() == nil || x.Name().Defn != stmt -> !ir.DeclaredBy(x, stmt)
}
'

Change-Id: I222a757296dbcb5d0889d617d221a9d7319f2d74
Reviewed-on: https://go-review.googlesource.com/c/go/+/275306
Reviewed-by: Russ Cox <rsc@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>