[Reassociate] Convert `add`-like `or`'s into an `add`'s to allow reassociation

InstCombine is quite aggressive in doing the opposite transform,
folding `add` of operands with no common bits set into an `or`,
and that not many things support that new pattern..

In this case, teaching Reassociate about it is easy,
there's preexisting art for `sub`/`shl`:
just convert such an `or` into an `add`:
https://rise4fun.com/Alive/Xlyv