[AVX512] adding AVXVBMI feature flag

The feature flag is for VPERMB,VPERMI2B,VPERMT2B and VPMULTISHIFTQB instructions. 
More about the instruction can be found in:
hattps://software.intel.com/sites/default/files/managed/07/b7/319433-023.pdf

Differential Revision: http://reviews.llvm.org/D16190

llvm-svn: 258012