[SystemZ] Simplify f128 atomic load/store (#90977)

Change definition of expandBitCastI128ToF128 and expandBitCastF128ToI128
to allow for simplified use in atomic load/store.

Update logic to split 128-bit loads and stores in DAGCombine to also
handle the f128 case where appropriate. This fixes the regressions
introduced by recent atomic load/store patches.