in case regClass reg of
RcInteger -> MOV II64 (OpReg reg) (OpAddr (spRel off_w))
RcDouble -> MOV FF64 (OpReg reg) (OpAddr (spRel off_w))
+ RcFloat -> panic "mkSpillInstr/RcFloat"
-- ToDo: will it work to always spill as a double?
-- does that cause a stall if the data was a float?
#else