op.c: Null list and pushmark in void cx

See 7d3c8a6837 for the explanation.

‘our($foo,$bar);’ is one example of the legitimate use of list+push-
mark in void context.

Here, we can null them and remove them from the execution order, since
they are not necessary.

We only do this if this is an op sequence that padrange would not
optimise, since padrange needs a pushmark, and we don’t want to
disable it.

This combined with other recent optimisations makes code like this:

    our ($foo,@bar,%baz);
    our $a;
    our ($b,$c);
    die;

skip straight over all those variable declarations at run time, going
straight to the ‘die’.