Merge branch 'fix-priority-for-noop' into 'master'

Also fix default priority for NoOpInstruction

The default priority is 0 not None.
Again, this could be fixed in NoOpInstruction, but I consider
this fix to be the better one.

I just wanted to push this on the old branch, but I missed you by some minutes...
Btw, is NoOpInstruction fully implemented or is this WIP (BarrierInstruction definitely seems WIP)?

See merge request !7