[dev.regabi] cmd/compile: ir.Node is no longer an ssa.Aux

[dev.regabi] cmd/compile: ir.Node is no longer an ssa.Aux

After the previous rewrite, we can now remove CanBeAnSSASym and
CanBeAnSSAAux from the generic Node interface, and declare them just
on *ir.Name.

Updates #42982.

Change-Id: I865771fd30c95c009740410844f20ade08648343
Reviewed-on: https://go-review.googlesource.com/c/go/+/275790
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>
Reviewed-by: Russ Cox <rsc@golang.org>