[OpenMP] Merge barrier elimination into AAExecutionDomain

With this patch we track aligned barriers in AAExecutionDomain and also
delete unnecessary barriers there. This allows us to eliminate barriers
across blocks, across functions, and in the presence of complex accesses
that do not force a barrier. Further, we can use the collected
information to enable store-load forwarding in a threaded environment
(follow up patch).

Differential Revision: https://reviews.llvm.org/D140463