cmd/compile: hard fail if n.Opt() is not nil in walkCheckPtrArithmetic

n.Opt() is used in walkCheckPtrArithmetic to prevent infinite loops. The
fact that it's used today because n.Opt() is not used for OCONVNOP
during walk.go. If that changes, then it's not safe to repalce it
anymore. So doing hard fail if that case happens, the author of new
changes will be noticed and must change the usage of n.Opt() inside
walkCheckPtrArithmetic, too.

Change-Id: Ic7094baa1759c647fc10e82457c19026099a0d47
Reviewed-on: https://go-review.googlesource.com/c/go/+/202497
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/walk.go

@@ -3965,8 +3965,12 @@ func walkCheckPtrArithmetic(n *Node, init *Nodes) *Node {
 	// Calling cheapexpr(n, init) below leads to a recursive call
 	// to walkexpr, which leads us back here again. Use n.Opt to
 	// prevent infinite loops.
-	if n.Opt() == &walkCheckPtrArithmeticMarker {
+	if opt := n.Opt(); opt == &walkCheckPtrArithmeticMarker {
 		return n
+	} else if opt != nil {
+		// We use n.Opt() here because today it's not used for OCONVNOP. If that changes,
+		// there's no guarantee that temporarily replacing it is safe, so just hard fail here.
+		Fatalf("unexpected Opt: %v", opt)
 	}
 	n.SetOpt(&walkCheckPtrArithmeticMarker)
 	defer n.SetOpt(nil)