Enable AVX512_BF16 instructions, which are supported for BFLOAT16 in Cooper Lake

Summary:
1. Enable infrastructure of AVX512_BF16, which is supported for BFLOAT16 in Cooper Lake;
2. Enable VCVTNE2PS2BF16, VCVTNEPS2BF16 and DPBF16PS  instructions, which are Vector Neural Network Instructions supporting BFLOAT16 inputs and conversion instructions from IEEE single precision.
VCVTNE2PS2BF16: Convert Two Packed Single Data to One Packed BF16 Data.
VCVTNEPS2BF16: Convert Packed Single Data to Packed BF16 Data.
VDPBF16PS: Dot Product of BF16 Pairs Accumulated into Packed Single Precision.
For more details about BF16 isa, please refer to the latest ISE document: https://software.intel.com/en-us/download/intel-architecture-instruction-set-extensions-programming-reference

Author: LiuTianle

Reviewers: craig.topper, smaslov, LuoYuanke, wxiao3, annita.zhang, RKSimon, spatel

Reviewed By: craig.topper

Subscribers: kristina, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60550

llvm-svn: 360017