This directory contains the main IR used by the MLRISC optimizer.
The IR is mainly concerned with control flow and control dependence.
The IR contains the CFG, (post-)dominator tree, control depenence graph,
loop (interval) structure, plus other facets.  

This IR is quite different from the cluster datatype, which is used by the basic
MLRISC system.  In any case the cluster and the CFG are interconvertible.
