[AVX-512] Support ADD/SUB/MUL of mask vectors

Summary:
Currently we expand and scalarize these operations, but I think we should be able to implement ADD/SUB with KXOR and MUL with KAND.

We already do this for scalar i1 operations so I just extended it to vectors of i1.

Reviewers: zvi, delena

Reviewed By: delena

Subscribers: guyblank, llvm-commits

Differential Revision: https://reviews.llvm.org/D28888

llvm-svn: 292474