Restructure main code.
Add docstring for number_tmps().
Elaborate Graph.block_name().
Factor out getting instruction id to a method.
Implement explicit control flow edges.
Implement stable numbering for blocks and instructions.

For this, use on overlay deictionary to cache made up names.
Add way to render analyzed control flow among diconnected dataflow DAGs.

So, this option is not representation of LLVM IR as it is, but already result
of some transformation for it. But it's rather useful to understand IR
structure, so added here.
Start refactoring rendering options and adding optparse interface for them.
Add --block-edges and makes sure that edges indeed between clusters.

Both their heads and tails specifically.
Describe --block-edges
Add --block-edges-helpers options to insert invisible nodes.
These if's don't appear to be relevant.