[dev.regabi] cmd/compile: introduce FwdRefAux for wrapping ir.Node as ssa.Aux
OpFwdRef is the only SSA value that needs the ability to store an
arbitrary ir.Node in its Aux field. Every other SSA value always uses
an *ir.Name.
This CL introduces FwdRefAux, which wraps an ir.Node and implements
the ssa.Aux tag interface, so that a subsequent refactoring can change
ir.Node to not implement ssa.Aux.
Passes buildall w/ toolstash -cmp.
Updates #42982.
Change-Id: Id1475b28847579573cd376e82f28761d84cd1c23
Reviewed-on: https://go-review.googlesource.com/c/go/+/275788
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com> Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>