[SystemZ] Don't lower ATOMIC_LOAD/STORE to LOAD/STORE (#75879)

- Instead of lowering float/double ISD::ATOMIC_LOAD / ISD::ATOMIC_STORE
nodes to regular LOAD/STORE nodes, make them legal and select those nodes
properly instead. This avoids exposing them to the DAGCombiner.

- AtomicExpand pass no longer casts float/double atomic load/stores to integer
  (FP128 is still casted).