Additional information about edges.
Structure for recording edge equivalences as well as any pending
edge redirections during the dominator optimizer.
Computing and storing the edge equivalences instead of creating
them on-demand can save significant amounts of time, particularly
for pathological cases involving switch statements.
These structures live for a single iteration of the dominator
optimizer in the edge's AUX field. At the end of an iteration we
free each of these structures and update the AUX field to point
to any requested redirection target (the code for updating the
CFG and SSA graph for edge redirection expects redirection edge
targets to be in the AUX field for each edge.
