[llvm] r301037 - AArch64: add test for "fence singlethread"

[Tim Northover via llvm-commits]

> 
> Out of curiosity, why is this supposed to lower to two consecutive
> `dmb` instructions instead of one? (at least you're checking for the
> absence of two `dmb`).
> (I'm also not sure FileCheck can currently distinguish between the
> absence of a single `dmb` or two, when they're consecutive).

Just one DMB, but if it was hypothetically emitted it could come before or after the comment thing that I *am* looking for so I need 2 CHECK-NOTs.

Cheers.

Tim.