Add pass generate per block in a function a GraphViz Dot graph with ops as nodes

* Add GraphTraits that treat a block as a graph, Operation* as node and use-relationship for edges;
  - Just basic graph output;
* Add use iterator to iterate over all uses of an Operation;
* Add testing pass to generate op graph;

This does not support arbitrary operations other than function nor nested regions yet.

PiperOrigin-RevId: 268121782