Unnecessary `this` tempvar-ing

```haxe
class Main {
static public function main() {}

function doSomething() {
use(() -> test());
}

@:pure(false) function test() {}

@:pure(false) static function use(fn:()->Void) {}
}
```
```
$ haxe -main Main -D analyzer-optimize -D dump=pretty
```
Dump for `doSomething`:
```
function doSomething() {
var _gthis = this;
Main.use(function() {
_gthis.test();
});
}
```