When checking code like the following:
```
<?php
function checkNegated(string $key): void {
$arr = [
0 => "foo",
1 => "bar",
];
if (!array_key_exists($key, $arr)) {
printf("not found\n");
}
}
function check(string $key): void {
$arr = [
0 => "foo",
1 => "bar",
];
if (array_key_exists($key, $arr)) {
printf("found\n");
}
}
```
the `if` in `checkNegated` would cause:
```
ERROR: RedundantCondition - 9:10 - Type string for $key is never =int(0)
```
This happens when the array keys are all int literals, but the "needle"
is a string.
`array_key_exists()` uses a loose equality comparison, but the generated
assertions for this specific case
(`AssertionFinder::getArrayKeyExistsAssertions`) was generating strict
equality clauses. This commit fixes it by changing the generated clause
from `=` to `~`.
