Optimize vector select from all 0s or all 1s

As packed comparisons in AVX/SSE produce all 0s or all 1s in each SIMD lane,
vector select could be simplified to AND/OR or removed if one or both values
being selected is all 0s or all 1s.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179267 91177308-0d34-0410-b5e6-96231b3b80d8