[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/compile/internal/ssa/cse_test.go

@@ -14,6 +14,8 @@ type tstAux struct {
 	s string
 }
 
+func (*tstAux) CanBeAnSSAAux() {}
+
 // This tests for a bug found when partitioning, but not sorting by the Aux value.
 func TestCSEAuxPartitionBug(t *testing.T) {
 	c := testConfig(t)