message: refinement check: simplify formula if pre_src == pre_tgt
diff: @@ -275,6 +275,10 @@ static void check_refinement(Errors &errs, Transform &t,
  expr pre_src = src_state.getPre()();
  expr pre_tgt = tgt_state.getPre()();
  + // optimization: rewrite "tgt /\ (src -> foo)" to "tgt /\ foo" if src = tgt
  + if (pre_src.eq(pre_tgt))
  + pre_src = true;
  +
  auto [poison_cnstr, value_cnstr] = type.refines(src_state, tgt_state, a, b);
  expr memory_cnstr
  = src_state.returnMemory().refined(tgt_state.returnMemory());
  