[NFC][Reassociate] Add patterns where `or` is part of load reduction idiom

As Wei Mi is reporting in post-commit review:
  https://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20201116/853479.html
teaching -reassociate about add-like-or's (70472f34) results in breaking apart
load widening patterns, and reassociating them.

While that's great, it prevents the actual load widening in backend,
and that is not good. We should have load widening in middle-end,
but for now we should at least not regress the naive patterns..