Move the flow graph computation into a compute method which can be
called with multiple functions.
This allows us to reuse the ControlFlowGraph memory and keep an instance
in the Context.
Also rework the algorithm to be more robust against unreachable blocks.
- Add an is_reachable(ebb) method.
- Change idom(ebb) to just return an instruction.
- Make idom() return None for the entry block as well as unreachable
blocks.
