Fix swift-atomics testcase

This testcase was not testing what it meant: because there were only two checks for
dmb {{ish}} in the second function, it could have missed a bug where one of the three
required dmb {{ish}} became dmb {{ishst}}. As I was fixing it, I also added
CHECK-LABELs to make it a bit less brittle.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218341 91177308-0d34-0410-b5e6-96231b3b80d8