8231198: Shenandoah: heap walking should visit all roots most of the time

Reviewed-by: zgu, rkennke

Author: shipilev

src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp

@@ -1296,7 +1296,10 @@ void ShenandoahHeap::object_iterate(ObjectClosure* cl) {
   ShenandoahHeapIterationRootScanner rp;
   ObjectIterateScanRootClosure oops(&_aux_bit_map, &oop_stack);
 
-  if (unload_classes()) {
+  // If we are unloading classes right now, we should not touch weak roots,
+  // on the off-chance we would evacuate them and make them live accidentally.
+  // In other cases, we have to scan all roots.
+  if (is_evacuation_in_progress() && unload_classes()) {
     rp.strong_roots_do(&oops);
   } else {
     rp.roots_do(&oops);