Improve void varop optimisation

After eliding the varop, the optimisation added in 5afbd733 repro-
cesses the previous op if it is a nextstate op.  But it was doing this
by setting the current op to the one before the nextstate, so that the
o=o->op_next in the loop header would cause it to reprocess the next-
state in the next iteration.  So, if that nextstate op were at the
beginning of a subroutine, the optimisation would be skipped, and this
would still execute two nextstate ops in a row:

sub foo {
    our($a, $b);
    die;
}

So, instead, just use ‘goto’ to reprocess the op, and we can do it
even if there is no op before the nextstate.
[perl #122995] Hang with while(1) in a sub-list

It was hanging at compile time in some cases, e.g.:

sub foo { () = ($a, my $b, ($c, do { while(1) {} })) }

The optimisation added in 5.20 to turn list+pushmark into null ops
when they are in list context (effectively making ($a,($b,$c)) equiva-
lent to ($a,$b,$c) with regard to which ops are executed) followed
op_next pointers to find the last op that was a kid of the sublist.

You can’t just follow op_next pointers like that, because it will loop
at compile time on infinite loops like while (1){}.

In this case, the last kid was being found in order to elide the erst-
while list op from the op_next chain, but that is not necessary, since
later OP_NULL handling takes care of it anyway.