Improve void varop optimisation

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.