optimizer: don't move expressions into a with-continuation-mark

... unless the optimizer can prove that the expression doesn't
inspect continuation marks.

pkgs/racket-pkgs/racket-test/tests/racket/optimize.rktl

@@ -2248,7 +2248,17 @@
                (list (c? (c-q (c 1 2 3))))
                5)))
 
-
+(test-comp `(lambda (b)
+              (let ([v (unbox b)])
+                (with-continuation-mark 'x 'y (unbox v))))
+           `(lambda (b)
+              (with-continuation-mark 'x 'y (unbox (unbox b))))
+           #f)
+(test-comp `(lambda (b)
+              (let ([v (box b)])
+                (with-continuation-mark 'x 'y (box v))))
+           `(lambda (b)
+              (with-continuation-mark 'x 'y (box (box b)))))
 
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; Check splitting of definitions

racket/src/racket/src/optimize.c

@@ -3252,6 +3252,11 @@ static Scheme_Object *optimize_wcm(Scheme_Object *o, Optimize_Info *info, int co
 
   v = scheme_optimize_expr(wcm->val, info, 0);
 
+  /* The presence of a key can be detected by other expressions,
+     to increment vclock to prevent expressions incorrectly
+     moving under the mark: */
+  info->vclock++;
+
   b = scheme_optimize_expr(wcm->body, info, scheme_optimize_tail_context(context));
 
   if (omittable_key(k, info)