[PatternMatch] create and use matcher for 'not' that excludes undef elements

We needed a stricter version of m_Not for D114462, but I wasn't
sure if that was going to be required anywhere else, so I didn't bother
to make that reusable.

It turns out we have one more existing simplification that needs
this (currently miscompiles):
https://alive2.llvm.org/ce/z/9-nTKi

And there's at least one more fold in that family that we could add.

Differential Revision: https://reviews.llvm.org/D114882