[X86][AVX512] Adding new LLVM TableGen backend which generates the EVEX2VEX compressi...

[X86][AVX512] Adding new LLVM TableGen backend which generates the EVEX2VEX compressing tables.

X86EvexToVex machine instruction pass compresses EVEX encoded instructions by replacing them with their identical VEX encoded instructions when possible.
It uses manually supported 2 large tables that map the EVEX instructions to their VEX ideticals.
This TableGen backend replaces the tables by automatically generating them.

Differential Revision: https://reviews.llvm.org/D30451

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@297127 91177308-0d34-0410-b5e6-96231b3b80d8