[mlir][arith] Fold `select` with poison

If either of the operands of `select` is fully poisoned we can simply return the other.
This PR implements this optimization inside the `fold` method.

Note that this patch is the first to add a dependency on the UB dialect within Arith. Given this was inevitable (and part of the motivation) it should be fine I believe.

Differential Revision: https://reviews.llvm.org/D158986