Peephole optimise adjacent pairs of nextstate ops.

Peephole optimise adjacent pairs of nextstate ops.

Previously, in code such as

    use constant DEBUG=>0;

    sub GAK {
        warn if DEBUG;
        print "stuff\n";
    }

the ops for C<warn if DEBUG;> would be folded to a null op (ex-const), but
the nextstate op would remain, resulting in a runtime op dispatch of nextstate,
nextstate, ...

The execution of a sequence of nexstate ops is indistinguishable from just the
last nextstate op, so teach the peephole optimiser to eliminate the first of a
pair of nextstate ops. (Except where the first carries a label, as labels
mustn't be eliminated by the optimiser, and label usage isn't conclusively
known at compile time.)