[llvm] DAG: Use poison instead of undef in SimplifyDemandedVectorElts (PR #207788)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Just to warn you, creating ISD::POISON more often is unearthing a number of poison-unsafe x86 dag vector folds, I'm trying to clean them up but I need a bit more time.

https://github.com/llvm/llvm-project/pull/207788