More expressive filenames for graph-visualization

Currently the graph-visualisation keeps on overwriting the same file, `F.pdf` and `S.pdf`, which makes it tedious to visualize graphs for actual problems, where we have both a bilinear and linear form.

There are a few ways we could go about this, as the visualization happens in the IR.

1. Use the `form_name` as part of the output
2. Add explicit filename options.