message: fix crash when going through an unsupport llvm constexpr in phi
diff: @@ -1490,12 +1490,14 @@ public:
  = value_name(*i->getIncomingBlock(idx)) + "_" + phi_bb.getName();
  BB = &Fn.insertBBBefore(bridge, phi_bb);
  getBB(i->getIncomingBlock(idx)).replaceTargetWith(&phi_bb, BB);
  - auto op = get_operand(val);
  - assert(op);
  + if (auto op = get_operand(val)) {
  phi->addValue(*op, move(bridge));
  BB->addInstr(make_unique<Branch>(phi_bb));
  continue;
  }
  + error(*i);
  + return {};
  + }
  if (auto op = get_operand(val)) {
  phi->addValue(*op, value_name(*i->getIncomingBlock(idx)));
  