[AVX512] Implement 512-bit and masked shufflelo and shufflehi intrinsics directly with __builtin_shufflevector and __builtin_ia32_select. Also improve the formatting of the AVX2 version.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@272452 91177308-0d34-0410-b5e6-96231b3b80d8