[llvm] r205409 - Remove duplicated DMB instructions

[Tim Northover]

>> I think that was a cue for Yi to take on the project himself! :)
>
> I'll look into that when I have time.

If you do, I think that performing it at the IR level (on "fence
acquire"/"fence release") would be more useful than continuing the
backend hack. Both because other targets would benefit at the same
time and because duplicated DMBs are far more likely than the manually
inserted DSB/ISB.

The analysis would also probably be quite a bit simpler.

Cheers.

Tim.