[AVX512] Add zero-masking variant to AVX512_masking multiclass

This completes one item from the todo-list of r215125 "Generate masking
instruction variants with tablegen".

The AddedComplexity is needed just like for the k variant.

Added a codegen test based on valignq.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215173 91177308-0d34-0410-b5e6-96231b3b80d8