[llvm] [DAGCombiner] Don't drop atomic property of original load. (PR #75626)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> It seems this patch actually shouldn't go in since it is probably not intentional to have DAGCombiner work on LOADs that are actually atomic. In other words, the MMO atomic state would generally be ignored with an ISD::LOAD, so if anything the SystemZ::lowerATOMIC_LOAD() should be improved.
> 
> Ok to close this?

I think this should still go ahead. I still think it's a good idea to get rid of ISD::ATOMIC_LOAD and migrate to using load with atomic operands. This is a code simplification anyway 

https://github.com/llvm/llvm-project/pull/75626