8335395: G1: Verification does not detect references into Free regions

Reviewed-by: ayang, iwalulya

src/hotspot/share/gc/g1/g1CollectedHeap.inline.hpp

@@ -253,6 +253,7 @@ inline bool G1CollectedHeap::is_obj_filler(const oop obj) {
 }
 
 inline bool G1CollectedHeap::is_obj_dead(const oop obj, const G1HeapRegion* hr) const {
+  assert(!hr->is_free(), "looking up obj " PTR_FORMAT " in Free region %u", p2i(obj), hr->hrm_index());
   if (hr->is_in_parsable_area(obj)) {
     // This object is in the parsable part of the heap, live unless scrubbed.
     return is_obj_filler(obj);

src/hotspot/share/gc/g1/g1HeapRegion.cpp

@@ -547,7 +547,10 @@ class G1VerifyLiveAndRemSetClosure : public BasicOopIterateClosure {
     }
 
     bool failed() const {
-      return !_is_in_heap || this->_g1h->is_obj_dead_cond(this->_obj, _vo);
+      return !_is_in_heap ||
+             // is_obj_dead* assume that obj is not in a Free region.
+             this->_g1h->heap_region_containing(this->_obj)->is_free() ||
+             this->_g1h->is_obj_dead_cond(this->_obj, _vo);
     }
 
     void report_error() {