message: fix crash with constexpr
diff: @@ -1542,8 +1542,9 @@ public:
  // patch phi nodes for recursive defs
  for (auto &[phi, i] : todo_phis) {
  - for (unsigned idx = 0, e = i->getNumIncomingValues(); idx != e; ++idx) {
  insert_constexpr_before = phi;
  + BB = const_cast<BasicBlock*>(&Fn.bbOf(*phi));
  + for (unsigned idx = 0, e = i->getNumIncomingValues(); idx != e; ++idx) {
  if (auto op = get_operand(i->getIncomingValue(idx))) {
  phi->addValue(*op, value_name(*i->getIncomingBlock(idx)));
  } else {
  