[Reassociate] Don't convert add-like-or's into add's if they appear to be part...

[Reassociate] Don't convert add-like-or's into add's if they appear to be part of load-combining 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.

For now, simply exclude any such `or` that appears to be a root of
load widening idiom from the or->add transformation.

Note that the heuristic is greedy, it doesn't ensure that loads
can *actually* be widened into a single load.