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.
Include the test file preamble comments when building a filecheck
instance for every function in the file.
This makes it possible to define common regex variables in the preamble
and use these definitions for all the functions.
