Elide our($foo) from execution order in void cx

our($foo) in void context has a compile-time affect but no run-item
effect, except to execute ops needlessly.  There is no need to execute
those ops.  We can simply remove them from the execution order.

‘our($foo,$fit,$far);’ still leaves a list and a pushmark, which I
plan to eliminate in the next commit.

‘our $foo; our $bar; our $baz;’ has no run-time cost now, because,
once the variables are eliminated, the nextstate ops in betwen them,
now adjacent, are also eliminated by another optimisation.