[llvm] [LLVM][SelectionDAG] Align poison/undef binop folds with IR. (PR #149334)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm approved this pull request.

LGTM with test nit. Don't use auto for types and you don't need to use the get* APIs for trivial types 

https://github.com/llvm/llvm-project/pull/149334