Implement llvm memory fence intrinsic.

[toddaaro]

It looks like we have almost every atomic intrinsic now, but we still don't seem to have the 'fence' intrinsic.

http://llvm.org/releases/3.0/docs/LangRef.html#i_fence

I believe this can be implemented fairly easily, but it requires some comfort manipulating the trans code. @Aatch has a recent pull request that does this in the appropriate areas. #7168

Blocks #4877

[Aatch]

I'm just testing a patch that adds fence intrinsics. Should be up soon.