[x86][inline-asm][AVX512][clang][PART-1] Introducing "k" and "Yk" constraints...

[x86][inline-asm][AVX512][clang][PART-1] Introducing "k" and "Yk" constraints for extended inline assembly, enabling use of AVX512 masked vectorized instructions.

Commit on behalf of mharoush

Extending inline assembly support, compatible with GCC as folowing:
 "k" constraint hints the compiler to select any of AVX512 k0-k7 registers.
 "Yk" constraint is a subset of "k" excluding k0 which is not allowd to be used as a mask.

Reviewer: 1. rnk

Differential Revision: https://reviews.llvm.org/D25063

llvm-svn: 285604