[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>
[AVX512] Add some of the FP cast intrinsics

Part of <rdar://problem/17688758>
[AVX512] Add intrinsic for knot

Part of <rdar://problem/17688758>