Make `iNext` smarter for mixed `▷` and `▷^n`

See below:

From iris Require Import proofmode.tactics.

Lemma foo {M} (P : uPred M) n : ▷ ▷^n P ⊢ ▷^n ▷ P.
Proof. iIntros "H". iNext.
(* Gives

"H" : ▷ ▷^n P
--------------------------------------∗
▷ P

*)