[AVX512] Add set1 intrinsics

(Dropped the byte and word variants from the patch.  Turns out these are not
part of AVX512F but only AVX512BW/VL.)

Part of <rdar://problem/17688758>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214314 91177308-0d34-0410-b5e6-96231b3b80d8
[AVX512] Add some of the FP cast intrinsics

Part of <rdar://problem/17688758>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214315 91177308-0d34-0410-b5e6-96231b3b80d8
[AVX512] Add intrinsic for knot

Part of <rdar://problem/17688758>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214316 91177308-0d34-0410-b5e6-96231b3b80d8