[SelectionDAG][X86] Improve legalization of v32i1 CONCAT_VECTORS of v16i1 for AVX512F.
A v32i1 CONCAT_VECTORS of v16i1 uses promotion to v32i8 to legalize the v32i1. This results in a bunch of extract_vector_elts and a build_vector that ultimately gets scalarized.
This patch checks to see if v16i8 is legal and inserts a any_extend to that so that we can concat v16i8 to v32i8 and avoid creating the extracts.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@320674
91177308-0d34-0410-b5e6-
96231b3b80d8
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