go/types, types2: pull up Unalias call to cover all of cycleFinder.typ

Without a test because it's unclear the situation can actually occur,
but the code is correct because it now mimics the behavior without
explicit Alias nodes.

For #67547.

Change-Id: I21a31af28880ca6d599fe465563d9574c26ed1f1
Reviewed-on: https://go-review.googlesource.com/c/go/+/588117
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/infer.go

@@ -693,6 +693,7 @@ type cycleFinder struct {
 }
 
 func (w *cycleFinder) typ(typ Type) {
+	typ = Unalias(typ)
 	if w.seen[typ] {
 		// We have seen typ before. If it is one of the type parameters
 		// in w.tparams, iterative substitution will lead to infinite expansion.
@@ -714,8 +715,8 @@ func (w *cycleFinder) typ(typ Type) {
 	case *Basic:
 		// nothing to do
 
-	case *Alias:
-		w.typ(Unalias(t))
+	// *Alias:
+	//      This case should not occur because of Unalias(typ) at the top.
 
 	case *Array:
 		w.typ(t.elem)