`add nuw i1` should simplify to `or disjoint i1` (not to `xor i1`)

[scottmcm]

Today, add nuw i1 optimizes to xor i1: https://llvm.godbolt.org/z/n5fY4x8Gx

That loses information, though. It could instead optimize to or disjoint i1 (https://alive2.llvm.org/ce/z/PUwk-C), to preserve the fact that it can't be true + true.