[llvm] llvm-reduce: Add reduceOperandsToPoison reduction (PR #132862)

[John Regehr via llvm-commits]

regehr wrote:

I'm not personally a fan of reducing to poison. this can make test cases smaller but the cognitive load of understanding poison is significant, so I'm not sure this is really a win

https://github.com/llvm/llvm-project/pull/132862