$primcall always continues to $kargs

* module/language/cps/devirtualize-integers.scm (peel-trace): Update
  comment.
* module/language/cps/contification.scm (apply-contification): Update to
  never contify a return of $primcall to $kreceive.
* module/language/cps/verify.scm (check-arities): Require that primcalls
  continue to $kargs.

module/language/cps/contification.scm

@@ -413,19 +413,14 @@ function set."
     (let ((k* (lookup-return-cont k)))
       (if (eq? k k*)
           (with-cps cps (build-term ($continue k src ,exp)))
-          ;; We are contifying this return.  It must be a call, a
-          ;; $primcall that can continue to $ktail (basically this is
-          ;; only "throw" and friends), or a $values expression.  k*
-          ;; will be either a $ktail or a $kreceive continuation.
+          ;; We are contifying this return.  It must be a call or a
+          ;; $values expression.  k* will be either a $ktail or a
+          ;; $kreceive continuation.
           (match (intmap-ref conts k*)
             (($ $kreceive ($ $arity req () rest () #f) kargs)
              (match exp
                (($ $call)
                 (with-cps cps (build-term ($continue k* src ,exp))))
-               ;; A primcall that can continue to $ktail can also
-               ;; continue to $kreceive.
-               (($ $primcall)
-                (with-cps cps (build-term ($continue k* src ,exp))))
                ;; We need to punch through the $kreceive; otherwise we'd
                ;; have to rewrite as a call to the 'values primitive.
                (($ $values vals)

module/language/cps/devirtualize-integers.scm

@@ -123,7 +123,8 @@ the trace should be referenced outside of it."
     (match (intmap-ref cps label)
       ;; We know the initial label is a $kargs, and we won't follow the
       ;; graph to get to $kreceive etc, so we can stop with these two
-      ;; continuation kinds.
+      ;; continuation kinds.  (For our purposes, only $values can
+      ;; continue to $ktail.)
       (($ $ktail) (fail))
       (($ $kargs names vars term)
        (let* ((vars-of-interest

module/language/cps/verify.scm

@@ -282,12 +282,7 @@ definitions that are available at LABEL."
        (assert-kreceive-or-ktail))
       (($ $primcall name param args)
        (match cont
-         (($ $kargs) #t)
-         ;; FIXME: Remove this case; instead use $prim and $call.
-         (($ $kreceive) #t)
-         (($ $ktail)
-          (unless (memv name '(throw throw/value throw/value+data))
-            (error "primitive should continue to $kargs, not $ktail" name)))))))
+         (($ $kargs) #t)))))
   (define (check-term term)
     (match term
       (($ $continue k src exp)