[LowerTypes] Fix a race condition (#4582)

This PR fixes a race condition in LowerTypes with aggregate preservation mode. In the instance op lowering, linkage of referenced module is checked, however it is not allowed to read attributes from different threads.

lib/Dialect/FIRRTL/Transforms/LowerTypes.cpp

@@ -451,8 +451,12 @@ TypeLoweringVisitor::getPreservatinoModeForModule(FModuleLike module) {
   // We cannot preserve external module ports.
   if (!isa<FModuleOp>(module))
     return PreserveAggregate::None;
+
+  // If `module` is a top-module, we have to lower ports. Don't read attributes
+  // of `module` since the attributes could be mutated in a different thread.
   if (aggregatePreservationMode != PreserveAggregate::None &&
-      preservePublicTypes && cast<hw::HWModuleLike>(*module).isPublic())
+      preservePublicTypes &&
+      module->getParentOfType<CircuitOp>().getMainModule(&symTbl) == module)
     return PreserveAggregate::None;
   return aggregatePreservationMode;
 }