[dev.regabi] cmd/compile: add ssa.Aux tag interface for Value.Aux

It's currently hard to automate refactorings around the Value.Aux
field, because we don't have any static typing information for it.
Adding a tag interface will make subsequent CLs easier and safer.

Passes buildall w/ toolstash -cmp.

Updates #42982.

Change-Id: I41ae8e411a66bda3195a0957b60c2fe8a8002893
Reviewed-on: https://go-review.googlesource.com/c/go/+/275756
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>

src/cmd/internal/obj/s390x/condition_code.go

@@ -124,3 +124,5 @@ func (c CCMask) String() string {
 	// invalid
 	return fmt.Sprintf("Invalid (%#x)", c)
 }
+
+func (CCMask) CanBeAnSSAAux() {}