Add two helper functions: isAtLeastAcquire, isAtLeastRelease

These methods are available on AtomicOrdering values, and will be used
in a later separate patch.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215779 91177308-0d34-0410-b5e6-96231b3b80d8