-- argRegs is the set of regs which are read for an n-argument call to C.
-- For archs which pass all args on the stack (x86), is empty.
-- Sparc passes up to the first 6 args in regs.
--- Dunno about Alpha.
argRegs :: RegNo -> [Reg]
argRegs 0 = []
argRegs 1 = map regSingle [3]