Fix number of values being broken by unwind-protect cleanup

The new direct values code neglected to reset the number of values
in the thread-local values, so whatever a cleanup did would be
kept in the event of a nonlocal exit. Reproducer:

(defun reproduce (cleanup)
  (block done
    (multiple-value-bind (body status headers)
        (values :body 200 :headers)
      (unwind-protect
           (return-from done
             (values-list (list body status headers :uri nil)))
        (funcall cleanup)))))

(loop for cleanup in
      (list (lambda () (values))
            (lambda () (values :one))
            (lambda () (values :one :two)))
      collect (multiple-value-list (reproduce cleanup)))

should return (:body 200 :headers) three times, did return
() (:body) (:body 200) as per the cleanup values.

This is all in native code obviously.

Author: Bike

src/lisp/kernel/cleavir/ir.lisp

@@ -221,6 +221,7 @@ And convert everything to JIT constants."
                          (%i1 0))))
 
 (defun load-all-values (nret storage)
+  (cmp::set-thread-nvalues nret)
   (%intrinsic-call "llvm.memcpy.p0.p0.i64"
                    (list (cmp::thread-return-values) storage
                          (cmp::irc-shl nret 3 :nuw t)