-;;;
-;;; The tricky part is doing this without trashing any of the calling
-;;; convention registers that are still needed. This vop is emitted
-;;; directly after the xep-allocate frame. That means the registers
-;;; are in use as follows:
-;;;
-;;; RAX -- The lexenv.
-;;; RBX -- Available.
-;;; RCX -- The total number of arguments.
-;;; RDX -- The first arg.
-;;; RDI -- The second arg.
-;;; RSI -- The third arg.
-;;;
-;;; So basically, we have one register available for our use: RBX.
-;;;
-;;; What we can do is push the other regs onto the stack, and then
-;;; restore their values by looking directly below where we put the
-;;; more-args.