[LICM] hoist fences out of loops w/o memory operations

The motivating case is an otherwise dead loop with a fence in it. At the moment, this goes all the way through the optimizer and we end up emitting an entirely pointless loop on x86. This case may seem a bit contrived, but we've seen it in real code as the result of otherwise reasonable lowering strategies combined w/thread local memory optimizations (such as escape analysis).

To handle this simple case, we can teach LICM to hoist must execute fences when there is no other memory operation within the loop.

Differential Revision: https://reviews.llvm.org/D50489

llvm-svn: 339378