ForbidVariableTypeOverwriting: relax allowed narrowing and generalization

[janedbal]

Since phpstan/phpstan-src#3853, following assignment:

interface SomeInterface {}
class SomeClass {}

function test(SomeInterface & SomeClass $classWithInterface) {
    $classWithInterface = new SomeClass();
}

no longer passes the old condition as those are false:

$previousVariableType->isSuperTypeOf($newVariableType)->yes(); // false
$newVariableType->isSuperTypeOf($previousVariableType)->yes(); // false

[janedbal]

Logic kept, superseded by #296