[llvm] DONT_MERGE: Reland "AtomicExpand: Allow incrementally legalizing atomicrmw" (PR #107309)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I think we can avoid any of the iterator complexity by just doing the instruction loop in reverse

But then that defeats the point

https://github.com/llvm/llvm-project/pull/107309