[PATCH] Add a fence elimination pass

[Daniel Berlin]

Hey, if you can get it to work fast with a min-cut formulation on programs
with lots of fences, that would be awesome :)
As I told JF, I haven't played with such formulations in about 7 years, so
it's entirely possible the world is a better place now.

They are certainly conceptually simpler to work with and reason about :)

http://reviews.llvm.org/D5758