[llvm] [VectorUtils] Pad concatenateVectors with poison instead of undef (PR #225598)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon requested changes to this pull request.

Make sure you commit the baseline tests as the first commit in the PR so it can the show the codegen diff in when you actually make the change in the later commit in the chain. The PR is squashed when committed to trunk

https://github.com/llvm/llvm-project/pull/225598