Fix constant-needs-allocation? for $values uses

* module/language/cps/dfg.scm (constant-needs-allocation?): Use of a
  constant in a $values expression of any arity does not cause slot
  allocation.

* module/language/cps/compile-bytecode.scm (compile-fun): Allow $values
  with a constant value to be compiled in test context.  Really we
  should fold these in a previous pass!

module/language/cps/compile-bytecode.scm

@@ -421,7 +421,17 @@
           (unless (eq? kf next-label)
             (emit-br asm kf)))))
       (match exp
-        (($ $values (sym)) (unary emit-br-if-true sym))
+        (($ $values (sym))
+         (call-with-values (lambda ()
+                             (lookup-maybe-constant-value sym allocation))
+           (lambda (has-const? val)
+             (if has-const?
+                 (if val
+                     (unless (eq? kt next-label)
+                       (emit-br asm kt))
+                     (unless (eq? kf next-label)
+                       (emit-br asm kf)))
+                 (unary emit-br-if-true sym)))))
         (($ $primcall 'null? (a)) (unary emit-br-if-null a))
         (($ $primcall 'nil? (a)) (unary emit-br-if-nil a))
         (($ $primcall 'pair? (a)) (unary emit-br-if-pair a))

module/language/cps/dfg.scm

@@ -846,7 +846,7 @@
          (lambda (use)
            (match (find-expression (lookup-cont use conts))
              (($ $call) #f)
-             (($ $values (_ _ . _)) #f)
+             (($ $values) #f)
              (($ $primcall 'free-ref (closure slot))
               (not (eq? sym slot)))
              (($ $primcall 'free-set! (closure slot value))