Update backend/cfg/simplify_terminator.ml

Co-authored-by: Xavier Clerc <xclerc@users.noreply.github.com>

backend/cfg/simplify_terminator.ml

@@ -101,6 +101,13 @@ let block (block : C.basic_block) =
   | Call_no_return _
   | Tailcall (Self _ | Func _)
   | Raise _ | Return | Poll_and_jump _ ->
+  (* CR xclerc: I wonder whether Merge_straightline_block should be updated
+to optimize a Poll_and_jump block where block is empty, and
+has a terminator such as Always block'.
+
+(Note that the blocks are currently and correctly not merged
+because Poll_and_jump block can raise. The optimization I
+suggest would be to rewrite it to Poll_and_jump block'.) *)
     ()
 
 let run cfg = C.iter_blocks cfg ~f:(fun _ b -> block b)