(* apply this tactic everywhere except inside match *)
Ltac destr c := match c with context [ ?_a =? ?_b ] => destruct (_a =? _b) end.

(* find occurrences of matches, avoid applying it inside the branches of the match *)
Ltac test_m z := lazymatch z with
  | context C [ match ?_a with _ => _ end ] => destr _a
  | _ => destr z
  end.

Parameter n m : nat.
Goal (if n =? m then match n with 5 => if 6 =? n then m else m+1 | _ => n end else 6) = if m =? 9 then 7 else 9.
repeat match goal with |- ?_Q => idtac _Q ; test_m _Q end.
(* didn't apply it on the right hand side of equality :(( *)

Ltac destr c := match c with context [ ?_a =? ?_b ] => destruct (_a =? _b) end.

Ltac test_m z := match z with
  | match ?_a with _ => _ end => idtac _a ; destr _a
  | ?_f ?_a => first [ test_m _f | test_m _a ]
  end.