RFR: Revert commented out Op_CompareAndSwapB/Op_CompareAndSwapS in aarch64.ad

[Roman Kennke]

>> Apparently I made a mistake and commented out Op_CompareAndSwapB and
>> Op_CompareAndSwapS in aarch64.ad. Let's revert it to upstream state.
> 
> I don't think that's sufficient. Given I made those changes upstream,
> let me take care of that.

Ok, sure. But it really might just be a merge mistake. Better that you
check it though. Thanks!

Roman