[llvm] [IR][FunctionAttrs] Clarify memory effects of atomics (PR #193768)

[Johannes Doerfert via llvm-commits]

jdoerfert wrote:

> (I'm not touching Attributor, which needs some major rework to properly infer memory effects.)

To make sure I understand the actual functional change. FuncAttrs now uses the new GetMemoryEffects function rather than I.mayWriteToMemory() (or read). And that new function will adjust the memory effects of atomics properly to encapsulate "non-local" effects. Is that correct? Is there another change I missed?

https://github.com/llvm/llvm-project/pull/193768