[llvm] r185339 - ARM: relax the atomic release barrier to "dmb ishst"

[Tim Northover]

> Specifically, a load before a release barrier is required to complete before
> a store after the barrier.

Bollocks, I'd managed to convince myself that's what a "dm ishst" was.
It should be reverted in r185356.

*Grumbles (yet again) about that blasted section of the ARM ARM*.

Thanks for looking at the commit.

Tim.