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.
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