B::Concise: Give sequence numbers for null ops

B::Concise was not giving sequence numbers to null ops even if they
were part of the execution order.  Ideally, there should be no null
ops in the execution order, but, if it did happen, B::Concise was not
helpful in debugging it.  In fact, its output was quite confusing:

1        <;> nextstate(main 1 -e:1) v ->-

You can’t tell which ops it’s pointing at and where the optimisations
need to be tweaked.

(See the thread starting at
<20141016220840.29774.qmail@lists-nntp.develooper.com>.)