6829013: G1: set the default value of G1VerifyConcMarkPrintRechable to false

Summary: Turn off G1VerifyConcMarkPrintReachable by default to minimize the amount of verbose output we generate by default.
Reviewed-by: jmasa

src/share/vm/gc_implementation/g1/g1_globals.hpp

@@ -71,7 +71,7 @@
   develop(intx, G1MarkingVerboseLevel, 0,                                   \
           "Level (0-4) of verboseness of the marking code")                 \
                                                                             \
-  develop(bool, G1VerifyConcMarkPrintReachable, true,                       \
+  develop(bool, G1VerifyConcMarkPrintReachable, false,                      \
           "If conc mark verification fails, print reachable objects")       \
                                                                             \
   develop(bool, G1TraceMarkStackOverflow, false,                            \

src/share/vm/gc_implementation/g1/heapRegion.cpp

@@ -718,7 +718,7 @@ void HeapRegion::verify(bool allow_dirty) const {
       vl_cl.failures()) {
     g1->concurrent_mark()->print_prev_bitmap_reachable();
   }
-  guarantee(!vl_cl.failures(), "should not have had any failures");
+  guarantee(!vl_cl.failures(), "region verification failed");
   guarantee(p == top(), "end of last object must match end of space");
 }