[llvm] AtomicExpand: Allow incrementally legalizing atomicrmw (PR #103371)

[Nico Weber via llvm-commits]

nico wrote:

This seems to break check-clang everywhere, eg http://45.33.8.238/linux/147013/step_6.txt

Please take a look and revert for now if it takes a while to fix.

https://github.com/llvm/llvm-project/pull/103371