Fix continuation stackslot scanning issue in Concurrent Scavenger

Concurrent Scavenger use SCAN_REASON_SHOULDREMEMBER scanning for
rememberedSet prune phase, fix continuation stackslot scanning issue
for rememberedSet prune phase to avoid the remembered flag has been
overwritten to reset.

we should break the stack walking after seeing this first time the flag
being set to true, but it's not in our control since the iteration loop
is in the stack walker. the related enhancement could be done in future
release.

Signed-off-by: Lin Hu <linhu@ca.ibm.com>

runtime/gc_glue_java/ScavengerDelegate.cpp

@@ -323,7 +323,7 @@ MM_ScavengerDelegate::doStackSlot(MM_EnvironmentStandard *env, omrobjectptr_t *s
 			}
 			break;
 		case SCAN_REASON_SHOULDREMEMBER:
-			*shouldRemember = scavenger->shouldRememberSlot(slotPtr);
+			*shouldRemember |= scavenger->shouldRememberSlot(slotPtr);
 			break;
 		}
 	}