RSSA

[Matthew Fluet]

> Matthew, if you could try pushing the
> changes into the x86-codegen, that would be great.

I'm fighting the CCall / CReturn distinction more than I'd like to.  In
particular, is there a strong reason to put the dst in the CReturn rather
than in the CCall?  For example, the Arith transfer has the dst in
transfer, rather than in return location.  I'd prefer to go with
eliminating the CReturn kind (or at least making it nullary) in Machine 
and go with:

     | CCall of {args: Operand.t vector,
                 dst: (Operand.t * Size.t) option,
		 prim: Prim.t,
		 return: Label.t (* must be nullary *)}