Nonlocal prompt returns cause all effects

* module/language/cps/effects-analysis.scm (expression-effects): Prompts
  cause &all-effects.  I tried to limit this change to CSE but it was
  actually LICM that was borked, so better to be conservative
* test-suite/tests/control.test ("escape-only continuations"): Add
  test.

module/language/cps/effects-analysis.scm

@@ -517,7 +517,10 @@ is or might be a read or a write to the same location as A."
     ((or ($ $fun) ($ $rec) ($ $closure))
      (&allocate &unknown-memory-kinds))
     (($ $prompt)
-     (&write-object &prompt))
+     ;; Although the "main" path just writes &prompt, we don't know what
+     ;; nonlocal predecessors of the handler do, so we conservatively
+     ;; assume &all-effects.
+     &all-effects)
     ((or ($ $call) ($ $callk))
      &all-effects)
     (($ $branch k exp)

test-suite/tests/control.test

@@ -103,7 +103,17 @@
                            (cons element prefix)))
                      '()
                      lst)))))
-      (prefix 'a '(0 1 2 a 3 4 5)))))
+      (prefix 'a '(0 1 2 a 3 4 5))))
+
+  (pass-if "loop only in handler"
+    (let ((n #f))
+      (let lp ()
+        (or n
+            (call-with-prompt 'foo
+              (lambda ()
+                (set! n #t)
+                (abort-to-prompt 'foo))
+              (lambda (k) (lp))))))))
 
 ;;; And the case in which the compiler has to reify the continuation.
 (with-test-prefix/c&e "reified continuations"