[PATCH] Add a fence elimination pass

[JF Bastien]

> I agree it might be a good thing to run it anyway on all targets, but
some tests (at least) on Power contain conditional jumps based on undef,
and SimplifyCFG makes a complete mess of them.

Would it be worth fixing the tests and turning on SimplifyCFG in a first
pass? Or as you suggested having a disable flag for this (icky...).

http://reviews.llvm.org/D5758