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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> It looks like this was somehow using the -O0 codepath when it wasn't supposed to.
> 

It specifically requests expanding to [cmpxchg for any FP atomic](https://github.com/llvm/llvm-project/blob/ad6558c334e5f6cd4b5c7e6d3ffe8d5158c540b0/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp#L27064)

The dubious assume-O0-means-fast-RA case is handled below 


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