[llvm] [PGO] Implement PGO counter promotion for atomic updates (PR #202487)

[Wael Yehia via llvm-commits]

w2yehia wrote:

> I am of the opinion that we should just fix this properly rather than doing a carve out specifically for PGO counters.

I agree and that was my first approach but gave up because it looked to be a disruptive change and I wasn't sure there's interest in the community for generalizing LoadStorePromoter to handle relaxed atomics. 
I can look into it again after this patch.

https://github.com/llvm/llvm-project/pull/202487