Add support for contingent variables

[muglug]

This (bad) code currently emits a PossiblyUndefinedVariable issue. It shouldn't, as Psalm should know that $a is defined whenever $do_thing is !empty.

$do_thing = rand(0, 10) > 5;
if ($do_thing) {
    $a = 1;
}
if ($do_thing) {
    $b = $a;
}

This code should still emit a PossiblyUndefinedVariable issue, because $do_thing is redefined.

$do_thing = rand(0, 10) > 5;
if ($do_thing) {
    $a = 1;
}
$do_thing = rand(0, 10) > 5;
if ($do_thing) {
    $b = $a;
}

[muglug]

Closing, won't support