[dce] regression

(one of) the reduced issue that @markknol reported early today:

```haxe
extern class C1 {
var v:C2;
}

class C2 {}
```

generated js with `-dce=full`
```js
// Generated by Haxe 4.0.0 (git build development @ 2844994)
(function () { "use strict";
var C2 = function() { };
})();
```

The breaking commit seems to be 0b3e1265b8c3921f449c4facfbaaa43e334d087b, though it might just surface another issue...