[llvm] [SelectionDAG] Fix D66309: Allow unordered atomics to have some optimizations done for them (PR #85589)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm requested changes to this pull request.

This should be split into different patches for the different transforms, with appropriate tests for each. The description should also be more specific; i.e. the part that's tested is for merging unordered atomic stress.

https://github.com/llvm/llvm-project/pull/85589