message: Fix GraphVisualizer source-blocks dump
  Summary: Handle linearized source blocks. It is too hard to pretend they are separate MIEs, so just make it one long entry.
diff: @@ -290,7 +290,19 @@ class CodeVisualizer : public TaggedBase {
  }
  void source_block(const SourceBlock& sb) {
  - m_output << MFLOW_SOURCE_BLOCK << " \"" << show(sb.src) << "\"@" << sb.id;
  + m_output << MFLOW_SOURCE_BLOCK;
  +
  + for (auto cur = &sb; cur != nullptr; cur = cur->next.get()) {
  + m_output << " \"" << show(cur->src) << "\"@" << cur->id;
  + for (const auto& val : cur->vals) {
  + m_output << " ";
  + if (val) {
  + m_output << val->val << "/" << val->appear100;
  + } else {
  + m_output << "N/A";
  + }
  + }
  + }
  }
  template <typename... Args>
  