RemoveUnusedBrs: optimize unreachable control flow mixed with side-effecting branches

[xuruiyang2002]

Currently wasm-opt cannot optimize unused branch complexed with side-effect operations, such as

 (func $_start
  (local $0 i32)
  (block $block
   (br_if $block
    (local.tee $0
     (i32.const 1)
    )
   )
   (br_if $block
    (i32.load
     (i32.const 0)
    )
   )
   (local.set $0
    (i32.const 0)
   )
  )
  (i32.store
   (i32.const 0)
   (local.get $0)
  )
 )

Actually, the whole block can be removed. However, O3 cannot optimize it (while O2 could).

Fixes: #7637

[xuruiyang2002]

BTW: fuzzing for about one hour, and no bad news.