-;;; MULTIPLE-VALUE-PROG1 is represented implicitly in IR1 by having a
-;;; the result code use result continuation (CONT), but transfer
-;;; control to the evaluation of the body. In other words, the result
-;;; continuation isn't IMMEDIATELY-USED-P by the nodes that compute
-;;; the result.
-;;;
-;;; In order to get the control flow right, we convert the result with
-;;; a dummy result continuation, then convert all the uses of the
-;;; dummy to be uses of CONT. If a use is an EXIT, then we also
-;;; substitute CONT for the dummy in the corresponding ENTRY node so
-;;; that they are consistent. Note that this doesn't amount to
-;;; changing the exit target, since the control destination of an exit
-;;; is determined by the block successor; we are just indicating the
-;;; continuation that the result is delivered to.
-;;;
-;;; We then convert the body, using another dummy continuation in its
-;;; own block as the result. After we are done converting the body, we
-;;; move all predecessors of the dummy end block to CONT's block.
-;;;
-;;; Note that we both exploit and maintain the invariant that the CONT
-;;; to an IR1 convert method either has no block or starts the block
-;;; that control should transfer to after completion for the form.
-;;; Nested MV-PROG1's work because during conversion of the result
-;;; form, we use dummy continuation whose block is the true control
-;;; destination.