Incorrect ArrayFirstLastRector fix (Can't use function return value in write context)

```php
<?php

$a = [1,2,3];
unset($a[array_key_last($a)]);

$b = [1,2,3];
unset(array_last($b));
```

In this example, rector suggests replacing `$a[array_key_last($a)]` with `array_last($b)`, however this doesn't actually work:
> PHP Fatal error:  Can't use function return value in write context

Rector 2.2.14