AVX-512: Load and Extended Load for i1 vectors

Implemented load+{sign|zero}_extend for i1 vectors
Fixed failures in i1 vector load.
Covered loading of v2i1, v4i1, v8i1, v16i1, v32i1, v64i1 vectors for KNL and SKX.

Differential Revision: http://reviews.llvm.org/D18737

llvm-svn: 265259