Assignment in a key of array destructuring is ignored

### Bug report

In this code, PHPStan misses the assignment to `$a`.
```php
[($a = 'foo') => $b] = ['foo' => 1];
```

`$a` is actually assigned.
https://3v4l.org/U593s

### Code snippet that reproduces the problem

https://phpstan.org/r/6cf214ae-c4f8-48a5-8d82-3efe7087e842

### Expected output

No error

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