bug#50290: Using `setf` on `map-elt` repeats evaluation for returning the value of `setf`

[Okamsn]

Hello,

When I do `(setf (map-elt map key) (my-func))`, it expands to

     (let* ((key key))
       (condition-case nil
           (with-no-warnings
             (map-put! map key (my-func) nil))
         (map-not-inplace
          (setq map (map-insert map key (my-func)))
          (my-func))))

The repetition of `(my-func)` raises warnings.  Compare with using
`(setf (alist-get key map) (my-func))`, where the result of `(my-func)`
is bound to `v` so that it is only called once:

     (let* ((p (if (and nil (not (eq nil 'eq)))
                   (assoc key map nil)
                 (assq key map)))
            (v (my-func)))
       (progn
         (if p
             (setcdr p v)
           (setq map (cons (setq p (cons key v))
                           map)))
         v))

This is on Emacs 28 with Map.el 3.1.