[X86] AVX512FP16 instructions enabling 1/6

1. Enable FP16 type support and basic declarations used by following patches.
2. Enable new instructions VMOVW and VMOVSH.

Ref.: https://software.intel.com/content/www/us/en/develop/download/intel-avx512-fp16-architecture-specification.html

Reviewed By: LuoYuanke

Differential Revision: https://reviews.llvm.org/D105263