[AVX-512] Use 'vnot' instead of 'not' in patterns involving vXi1 vectors.

This fixes selection of KANDN instructions and allows us to remove an extra set of patterns for KNOT and KXNOR.

Reviewers: delena, igorb

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D26134

llvm-svn: 285878