[DDG] Data Dependence Graph - Pi Block

    Summary:
    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.

    Authored By: bmahjour

    Reviewer: Meinersbur, fhahn, myhsu, xtian, dmgreen, kbarton, jdoerfert

    Reviewed By: Meinersbur

    Subscribers: ychen, arphaman, simoll, a.elovikov, mgorny, hiraditya, jfb, wuzish, llvm-commits, jsji, Whitney, etiotto, ppc-slack

    Tag: #llvm

    Differential Revision: https://reviews.llvm.org/D68827

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