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'punpckhwd' instructions when suitable rather than falling back to the generic algorithm. While we could canonicalize to these patterns late in the process, that wouldn't help when the freedom to use them is only visible during initial lowering when undef lanes are well understood. This, it turns out, is very important for matching the shuffle patterns that are used to lower sign extension. Fixes a small but relevant regression in gcc-loops with the new lowering. When I changed this I noticed that several 'pshufd' lowerings became unpck variants. This is bad because it removes the ability to freely copy in the same instruction. I've adjusted the widening test to handle undef lanes correctly and now those will correctly continue to use 'pshufd' to lower. However, this caused a bunch of churn in the test cases. No functional change, just churn. Both of these changes are part of addressing a general weakness in the new lowering -- it doesn't sufficiently leverage undef lanes. I've at least a couple of patches that will help there at least in an academic sense. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217752 91177308-0d34-0410-b5e6-96231b3b80d8
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