8329764: G1: Handle null references during verification first

Reviewed-by: stefank, iwalulya

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

@@ -621,14 +621,18 @@ class G1VerifyLiveAndRemSetClosure : public BasicOopIterateClosure {
 
   template <class T>
   void do_oop_work(T* p) {
-    if (_failures->count() >= G1MaxVerifyFailures) {
+    // Check for null references first - they are fairly common and since there is
+    // nothing to do for them anyway (they can't fail verification), it makes sense
+    // to handle them first.
+    T heap_oop = RawAccess<>::oop_load(p);
+    if (CompressedOops::is_null(heap_oop)) {
       return;
     }
 
-    T heap_oop = RawAccess<>::oop_load(p);
-    if (CompressedOops::is_null(heap_oop)) {
+    if (_failures->count() >= G1MaxVerifyFailures) {
       return;
     }
+
     oop obj = CompressedOops::decode_raw_not_null(heap_oop);
 
     LiveChecker<T> live_check(_failures, _containing_obj, p, obj, _vo);