This patch adds Pi Blocks to the DDG. A pi-block represents a group of DDG nodes that are part of a strongly-connected component of the graph. Replacing all the SCCs with pi-blocks results in an acyclic representation of the DDG. For example if we have:

{a -> b}, {b -> c, d}, {c -> a}

the cycle `a -> b -> c -> a` is abstracted into a pi-block "p" as follows:

{p -> d} with "p" containing: {a -> b}, {b -> c}, {c -> a}

In this implementation the edges between nodes that are part of the pi-block are preserved. The crossing edges (edges where one end of the edge is in the set of nodes belonging to an SCC and the other end is outside that set) are replaced with corresponding edges to/from the pi-block node instead.