[OpenMP][FIX] Restrict more unsound assmptions about threading

Even if all loads and stores are in `nosync` functions we cannot
guarantee there is no synchronization going on between them. As such, we
cannot use CFG reasoning. We could check the entire module, or, what
happens now to minimize test churn, is to check if all accesses are in
the same function that is `nosync`. A follow up will undo some of the
regressions where possible.

Similarly, reachability cannot be used to exclude an access if the
access is not known to be executed by the same thread as the given
instruction.

The OpenMP-opt test was added for the latter problem.