Some recursive Tree::Optimizer passes appear to shed PAST::Stmts.

[ekiru]

This is the apparent source of the test failure at https://github.com/parrot/tree-optimization/blob/master/t/tree-optimizer.t#L214

It turns (Data::Dumper output)

"VAR1" => PMC 'PAST;Stmts'  {
    [0] => PMC 'PAST;Val'  {
        <value> => 6
    }
}

into

"VAR1" => PMC 'PAST;Val'  {
    <value> => 14
}

Precise bug not yet identified.