NCG: Refactor representation of code with liveness info

NCG: Refactor representation of code with liveness info

 * I've pushed the SPILL and RELOAD instrs down into the
   LiveInstr type to make them easier to work with.

 * When the graph allocator does a spill cycle it now just
   re-annotates the LiveCmmTops instead of converting them
   to NatCmmTops and back.

 * This saves working out the SCCS again, and avoids rewriting
   the SPILL and RELOAD meta instructions into real machine
   instructions.