[llvm] [LV][NFC] Change undef to poison in most tests (PR #163525)

[Kerry McLaughlin via llvm-commits]

https://github.com/kmclaughlin-arm approved this pull request.

LGTM, it makes sense to look at the tests which do change as a result of using poison separately.

https://github.com/llvm/llvm-project/pull/163525