[llvm] [AtomicExpand][NFC] Rename AtomicExpansionKind::Expand to AtomicExpansionKind::Xchg (PR #202825)

[Matt Arsenault via llvm-commits]

arsenm wrote:

I think this should stay as is and matches the usual legalization operation naming. The fact that we have a parallel legalization action system for atomics is already a wart on codegen. This should be the catch all and not require targets to know the details. The fact that this happens to just map to one case isn't a reason to expose that. If you want to change a name, CustomExpand -> Custom. 

https://github.com/llvm/llvm-project/pull/202825