[x86][inline-asm] Introducing (AVX512) k0-k7 registers for inline-asm usage

Commit on behalf of mharoush 

After LGTM and check all: 
This patch enables usage of k registers in inline assembly syntax.

Reviewer: 1. rnk
          2. delena 

Differential Revision: https://reviews.llvm.org/D25011

llvm-svn: 285555