[PATCH] Add a fence elimination pass

[JF Bastien]

(Discussing this offline with Danny)

I'd like measurements for your approach. I'm not sure it'll be slow on big functions because your graph is a subset of all the IR in the function.

As we discussed, csmith should be able to generate big functions with lots of fences. Measuring runtime at different CFG size and fence+surrounding memory size would be useful, and basic profiling would show where overheads are in the current code.

I think that should inform whether another algorithm should be tried.

http://reviews.llvm.org/D5758