[llvm] r185527 - ARM: relax the atomic release barrier to "dmb ishst" on Swift

[Tim Northover]

> In general, when adding these optimizations, can you try to link to
> the documentation that justifies the change?

Tricky, particularly in cases like this. Virtually no-one makes
microarchitectural documentation public these days because of the
patent trolls.

For things that are truly architectural, at least on ARM, the standard
reference is the "ARM Architecture Reference Manual", which has
standing links (alongside ABIs etc) in docs/CompilerWriterInfo.rst.

I suppose a mention of the section number in the commit message
wouldn't be out of place though in the more esoteric cases. I'll try
to remember in future.

Tim.