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36ad61f4ea
essentially a DAG combine that never gets a chance to run. We might typically expect DAG combining to remove shuffles-of-splats and other similar patterns, but we don't get a chance to run the DAG combiner when we recursively form sub-shuffles during the lowering of a shuffle. So instead hand-roll a really important combine directly into the lowering code to detect shuffles-of-splats, especially shuffles of an all-zero splat which needn't even have the same element width, etc. This lets the new vector shuffle lowering handle shuffles which implement things like zero-extension really nicely. This will become even more important when I wire the legalization of zero-extension to vector shuffles with the new widening legalization strategy. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212444 91177308-0d34-0410-b5e6-96231b3b80d8