message: fix leftover crash in phi handling after constexpr changes
diff: @@ -146,6 +146,9 @@ class llvm2alive_ : public llvm::InstVisitor<llvm2alive_, unique_ptr<Instr>> {
  "%__copy_" + to_string(copy_idx++), *ag,
  UnaryOp::Copy);
  auto val = v.get();
  + if (insert_constexpr_before)
  + BB->addInstrAt(std::move(v), insert_constexpr_before, true);
  + else
  BB->addInstr(std::move(v));
  return val;
  }
  