A powerful constant propagator and folder based on an algorithm
sketched by Cytron et al that takes conditional control flow into
account. This optimization demonstrates some of the benefits of
SSA -- particularly the fact that Phi nodes represent natural merge
points in the control flow.
author: Navindra Umanee See Also:Efficiently
* Computing Static Single Assignment Form and the Control Dependence
* Graph
Propagates constants to the definition and uses of the relevant
locals given a mapping. Notice that we use the Shimple
implementation of LocalDefs and LocalUses.