AVX-512: Truncating store for i1 vectors

Implemented truncstore for KNL and skylake-avx512.
Covered vectors from v2i1 to v64i1. We save the value in bits (not in bytes) - v32i1 is saved in 4 bytes.

Differential Revision: http://reviews.llvm.org/D18740

llvm-svn: 265283