SelectionDAG: skip barriers for unordered atomic operations

Unordered is strictly weaker than monotonic, so if the latter doesn't have any
barriers then the former certainly shouldn't.

rdar://problem/16548260

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209901 91177308-0d34-0410-b5e6-96231b3b80d8