Fix $x = (bool)$x; for undefined with opcache

Fix $x = (bool)$x; for undefined with opcache

And `$x = !$x`

Noticed while working on GH-4912

The included test would not emit undefined variable errors in php 8.0
with opcache enabled. The command used:

```
php -d zend_extension=opcache.so --no-php-ini -d error_reporting=E_ALL \
    -d opcache.file_cache= -d opcache.enable_cli=1  test.php
```