[X86][AVX512] Add support for masked shuffle comments

This patch adds support for including the avx512 mask register information in the mask/maskz versions of shuffle instruction comments.

This initial version just adds support for MOVDDUP/MOVSHDUP/MOVSLDUP to reduce the mass of test regenerations, other shuffle instructions can be added in due course.

Differential Revision: http://reviews.llvm.org/D21953

llvm-svn: 274459