Simplify assertions generated by an array_key_exists check

[muglug]

There are three forms of array_key_exists that are useful for Psalm's inference:

array_key_exists('known_key', $array_with_unknown_keys);
array_key_exists('known_key', $array_shape_with_possibly_undefined_key);
array_key_exists($unknown_key, $array_shape_with_known_keys);

In the third example we were generating one more assertion than was necessary. This should simplify the calculation, and fix #8741