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

[Matt Arsenault via llvm-commits]

arsenm 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

The alternative for target extension types that don't support zero initializer is to do nothing, which is not a win 

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