Okay, I think I found the problem.  In case of a lambda lifted function
we weren't doing this part of cconv--convert-function:

    (dolist (arg args)
      (if (not (member (cons (list arg) parentform) cconv-captured+mutated))
          (if (assq arg new-env) (push `(,arg) new-env))
        (push `(,arg . (car-safe ,arg)) new-env)
        (push `(,arg (list ,arg)) letbind)))

so `params' wasn't handled properly.  I copied and adapted that code to
the corresponding place in cconv-convert, and it seems to work.  Please
review for sanity.