[ISel] Move handling of atomic loads from SystemZ to DAGCombiner (NFC). (#86484)

The folding of sign/zero extensions into an atomic load by specifying an
extension type is not target specific, and therefore belongs in the
DAGCombiner rather than in the SystemZ backend.

- Handle atomic loads similarly to regular loads by adding
AtomicLoadExtActions with set/get methods.
- Move SystemZ extendAtomicLoad() to DagCombiner.cpp.