message: Fix leak
  Summary: As title. Use a `unique_ptr`.
diff: @@ -286,7 +286,7 @@ RemoveNullcheckStringArg::Stats RemoveNullcheckStringArg::change_in_cfg(
  if (iter == transfer_map.end()) {
  continue;
  }
  - IRInstruction* new_insn = new IRInstruction(OPCODE_INVOKE_STATIC);
  + auto new_insn = std::make_unique<IRInstruction>(OPCODE_INVOKE_STATIC);
  if (iter->second.second) {
  // We could have params copied via intermediate registers.
  auto defs = env.get(insn->src(0));
  @@ -329,12 +329,12 @@ RemoveNullcheckStringArg::Stats RemoveNullcheckStringArg::change_in_cfg(
  ->set_srcs_size(2)
  ->set_src(0, insn->src(0))
  ->set_src(1, tmp_reg);
  - m.replace(cfg.find_insn(insn), {cst_insn, new_insn});
  + m.replace(cfg.find_insn(insn), {cst_insn, new_insn.release()});
  } else {
  new_insn->set_method(iter->second.first)
  ->set_srcs_size(1)
  ->set_src(0, insn->src(0));
  - m.replace(cfg.find_insn(insn), {new_insn});
  + m.replace(cfg.find_insn(insn), {new_insn.release()});
  }
  stats.null_check_insns_changed++;
  }
  