Seen in real code, although not sure it's really impactful.
Currently the fold is limited to the case where there is a single use of load,
which is a cast, which generally makes sense. But if originally have
several identical casts in different BB's, and don't decide to hoist
them into some one BB, we won't CSE them, and the fold won't happen..
Instead, let's be smart, and let's simply natively handle the case
where all uses of load are identical casts.