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7847229c31
This patch teaches the DAGCombiner how to fold a pair of shuffles according to rules: 1. shuffle(shuffle A, B, M0), B, M1) -> shuffle(A, B, M2) 2. shuffle(shuffle A, B, M0), A, M1) -> shuffle(A, B, M3) The new rules would only trigger if the resulting shuffle has legal type and legal mask. Added test 'combine-vec-shuffle-3.ll' to verify that DAGCombiner correctly folds shuffles on x86 when the resulting mask is legal. Also added some negative cases to verify that we avoid introducing illegal shuffles. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213001 91177308-0d34-0410-b5e6-96231b3b80d8