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BBVectorize would, except for loads and stores, always fuse instructions so that the first instruction (in the current source order) would always represent the low part of the input vectors and the second instruction would always represent the high part. This lead to too many shuffles being produced because sometimes the opposite order produces fewer of them. With this change, BBVectorize tracks the kind of pair connections that form the DAG of candidate pairs, and uses that information to reorder the pairs to avoid excess shuffles. Using this information, a future commit will be able to add VTTI-based shuffle costs to the pair selection procedure. Importantly, the number of remaining shuffles can now be estimated during pair selection. There are some trivial instruction reorderings in the test cases, and one simple additional test where we certainly want to do a reordering to avoid an unnecessary shuffle. llvm-svn: 167122
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