This is an alternative approach to fixing https://reviews.llvm.org/D93990#2666922
by teaching m_Undef to match vectors/aggrs with poison elements instead of hard-coding
it at SimplifyQuery::isUndefValue (D99853).
This patch also contains fixes in InstCombine files to use the m_Undef matcher instead
of isa<UndefValue>.
clang-tidy: warning: invalid case style for class 'undef_match' [readability-identifier-naming]
not useful