7035144: G1: nightly failure: Non-dirty cards in region that should be dirty (failures still exist...)

We should only undirty cards after we decide that they are not on a young region, not before. The fix also includes improvements to the verify_dirty_region() method which print out which cards were not found dirty.

Reviewed-by: johnc, brutisso

hotspot/src/share/vm/memory/modRefBarrierSet.hpp

@@ -100,12 +100,6 @@ public:
   // Pass along the argument to the superclass.
   ModRefBarrierSet(int max_covered_regions) :
     BarrierSet(max_covered_regions) {}
-
-#ifndef PRODUCT
-  // Verifies that the given region contains no modified references.
-  virtual void verify_clean_region(MemRegion mr) = 0;
-#endif
-
 };
 
 #endif // SHARE_VM_MEMORY_MODREFBARRIERSET_HPP