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

[James Y Knight via llvm-commits]

jyknight wrote:

See also @preames comment in commit a0dde7b8d16842005d6f5247dda6f17c77a5906f. I think the idea is to delete the support for atomic LoadSDNode, after moving SystemZ off of it.

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