Currently, any atomic store/load above unordered kills all information in the
analysis. With this patch, the analysis is only killed by a release store
followed by an acquire load, or a RMW, or a fence
(see reference in the comments for why that is correct)
This appear to only impact DSE and GVN, see tests for examples.
This fixes the second part of http://llvm.org/bugs/show_bug.cgi?id=17281