make op.c:S_find_and_forget_pmops() non-recursive

make op.c:S_find_and_forget_pmops() non-recursive

For every CV that's freed which has a shared optree (e.g. a closure
or between threads), the whole optree is walked looking for PMOPs.
Make that walk non-recursive.

Contrived code that triggers a stack overflow:

{
    my $outer;
    my $e = 'sub { $outer && '
            . join('&&', ('$x') x 100_000)
            . " }";
    #print $e, "\n";
    eval $e;
}

Even after this commit, that code still SEGVs due to a separate stack
blow in Perl_rpeep().