Weak relaxing of the constraints on atomics in MemoryDependencyAnalysis

Monotonic accesses do not have to kill the analysis, as long as the QueryInstr is not
itself atomic.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215942 91177308-0d34-0410-b5e6-96231b3b80d8
Answer to Philip Reames comments

- add check for volatile (probably unneeded, but I agree that we should be conservative about it).
- strengthen condition from isUnordered() to isSimple(), as I don't understand well enough Unordered semantics (and it also matches the comment better this way) to be confident in the previous behaviour (thanks for catching that one, I had missed the case Monotonic/Unordered).
- separate a condition in two.
- lengthen comment about aliasing and loads
- add tests in GVN/atomic.ll

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215943 91177308-0d34-0410-b5e6-96231b3b80d8