message: Fix `foreach_source_block`
  Summary: Actually run over coalesced blocks.
diff: @@ -190,7 +190,9 @@ inline void foreach_source_block(cfg::Block* b, const Fn& fn) {
  if (mie.type != MFLOW_SOURCE_BLOCK) {
  continue;
  }
  - fn(mie.src_block.get());
  + for (auto* sb = mie.src_block.get(); sb != nullptr; sb = sb->next.get()) {
  + fn(sb);
  + }
  }
  }
  template <typename Fn>
  @@ -199,7 +201,9 @@ inline void foreach_source_block(const cfg::Block* b, const Fn& fn) {
  if (mie.type != MFLOW_SOURCE_BLOCK) {
  continue;
  }
  - fn(mie.src_block.get());
  + for (auto* sb = mie.src_block.get(); sb != nullptr; sb = sb->next.get()) {
  + fn(sb);
  + }
  }
  }
  