[PATCH] Add a fence elimination pass

[Robin Morisset]

I missed your idea of running this pass for C++11 fences instead. In short: this is probably useless, as C++11 fences are expected to be rather rare (compared to atomic accesses that result in target-specific fences), and the semantics of C++11 fences are hairy enough I don't want to try without a clear benefit. So while the infrastructure could support it, I don't plan on doing it.

http://reviews.llvm.org/D5758