=> (LAOfloor
       ptr
       (SLW <typ.UInt32>
         (MOVBZreg <typ.UInt32> val)
         (RXSBG <typ.UInt32> {s390x.NewRotateParams(59, 60, 3)} (MOVDconst [3<<3]) ptr))
       mem)

(AtomicAnd32 ...) => (LAN ...)
(AtomicOr32  ...) => (LAO ...)

// Lowering extension
// Note: we always extend to 64 bits even though some ops don't need that many result bits.
(SignExt8to(16|32|64) ...) => (MOVBreg ...)