side project: include dataflow analysis/analyses in `--pretty flowgraph`

[pnkfelix]

Right now, if you are debugging a dataflow analysis, you can see the the results of the analysis via debug! by various settings of the RUST_LOG environment variable.

It might be nice to expand the --pretty flowgraph output to allow one to annotate the labels in the graphviz output with result of the dataflow analysis.

[pnkfelix]

I would be willing to mentor this.