[PATCH] [DagCombiner] Allow shuffles to merge through bitcasts

[Simon Pilgrim]

Thanks Andrea, I was able to include some of your minors in this patch. I'll be sending a follow up patch soon (probably next week now) that will remove all the code duplication we have for shuffle mask commutation.


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  rL LLVM

http://reviews.llvm.org/D7939

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