Stop `iAssumption` from unifying evar premises with `False`

Opening this so that I don't forget about it. The following example (thanks @Blaisorblade) should show the current, unwanted behavior so I should probably add this as a test case:

Lemma foo {M} : ⊢ ∃ (P : uPred M), P -∗ (∀ (x : nat), ⌜ x = x ⌝) ∗ P.
Proof.
  iExists ?[P]. (* [iExists _.] also works. *)
  iIntros "P". iSplit. 
  iIntros (x).
  Fail iApply "P".
  iAssumption.
  Show.
  (*
   M : ucmraT
  ============================
  "P" : False
  --------------------------------------∗
  False
  *)