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Previously, irreducible backedges were ignored. With this commit, irreducible SCCs are discovered on the fly, and modelled as loops with multiple headers. This approximation specifies the headers of irreducible sub-SCCs as its entry blocks and all nodes that are targets of a backedge within it (excluding backedges within true sub-loops). Block frequency calculations act as if we insert a new block that intercepts all the edges to the headers. All backedges and entries to the irreducible SCC point to this imaginary block. This imaginary block has an edge (with even probability) to each header block. The result is now reasonable enough that I've added a number of testcases for irreducible control flow. I've outlined in `BlockFrequencyInfoImpl.h` ways to improve the approximation. <rdar://problem/14292693> git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207286 91177308-0d34-0410-b5e6-96231b3b80d8
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