Update m_Undef to match vectors/aggrs with undefs and poisons mixed

This fixes https://reviews.llvm.org/D93990#2666922
by teaching `m_Undef` to match vectors/aggrs with poison elements.

As suggested, fixes in InstCombine files to use the `m_Undef` matcher instead
of `isa<UndefValue>` will be followed.

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D100122