[PatternMatch] define m_Not using m_Xor and cst_pred_ty

Using cst_pred_ty in the definition allows us to match vectors with undef elements.

This is a continuation of an effort to make all pattern matchers allow undef elements in vectors:
rL325437
rL325466
D43792

Differential Revision: https://reviews.llvm.org/D44076

llvm-svn: 326823