Fix condition-case body for-effect miscompilation

(condition-case x A (:success B)) should not compile A for-effect even
if the entire form is in for-effect context.

* lisp/emacs-lisp/byte-opt.el (byte-optimize-form-code-walker):
Don't optimise the condition-case body form for effect (potentially
discarding its value) if there is a success handler and a variable.
* test/lisp/emacs-lisp/bytecomp-tests.el (bytecomp-tests--test-cases):
Add test cases.

lisp/emacs-lisp/byte-opt.el

@@ -410,7 +410,10 @@ for speeding up processing.")
 
       (`(condition-case ,var ,exp . ,clauses)
        `(,fn ,var          ;Not evaluated.
-            ,(byte-optimize-form exp for-effect)
+             ,(byte-optimize-form exp
+                                  (if (assq :success clauses)
+                                      (null var)
+                                    for-effect))
           ,@(mapcar (lambda (clause)
                       (let ((byte-optimize--lexvars
                              (and lexical-binding