Add DataflowParent OpTag

[lmondada]

I propose adding an OpTag to gather all OpType whose children form a dataflow sibling graph. I've called this an IOBlock to refer to the property that they are guaranteed to have an Input and Output child.

Other possible names could be DataflowWrapper, DataflowParent, DataBlock... Opinions?

[cqc-alec]

Good idea. Not too keen on the name IOBlock, since it only really makes sense with detailed knowledge of the spec (one might assume that a CFG is also an I/O block). Maybe DataflowParent or simply Dataflow?

[lmondada]

I'm not sure what the definition of DataflowChild should be exactly. Right now OpTag::Dfg and OpTag::TailLoop are both DataflowChild and DataflowParent.