Fix union assertions

[danog]

Fixes #8322, #8319 and a few other bugs introduced by #8077, by explicitly running the intersection logic only on IsIdentical assertions (due to the semantics of Type::intersectUnionTypes wasn't sure if we should run it on IsType assertions), and removing the custom intersection/equality logic which broke other stuff in the code.

[danog]

The shepherd issue seems to be present upstream: https://psalm.dev/r/e0bee4534f

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/e0bee4534f

<?php
                    /**
                     * Asserts that two variables are the same.
                     *
                     * @param object $actual
                     * @psalm-assert-if-true (a|b|c) $actual
                     */
                    function assertSame($actual) : void {}
                    class a {}
                    class b {}
                    class c {}
/** @var object */
                    $actual = new c;
if (assertSame($actual)) {
/** @psalm-trace $actual */;
}

Psalm output (using commit 63b389f):

INFO: UnusedParam - 8:41 - Param $actual is never referenced in this method

INFO: Trace - 15:28 - $actual: a

[danog]

Mm actually even with the additional int range handling in intersections we're still missing intersection logic for keyed arrays, I feel like I'm duplicating code already present in the assertion reconciler, once I fix the last unit case I'd still prefer to get this merged, but eventually we should be using the same code used in the assertion reconciler to perform intersections, differences&so on...

[danog]

All done!

[orklah]

Thanks!