Speedup GlobalHitsThresholdChecker a little (#13836)

Even though this field is not `volatile`, writing it isn't free and
causes needless cache thrashing at some frequency. We can speed things
up by only writing the `true` value and never the `false` value.

lucene/core/src/java/org/apache/lucene/search/HitsThresholdChecker.java

@@ -46,7 +46,11 @@ boolean isThresholdReached() {
       if (thresholdReached) {
         return true;
       }
-      return thresholdReached = globalHitCount.longValue() > getHitsThreshold();
+      if (globalHitCount.longValue() > getHitsThreshold()) {
+        thresholdReached = true;
+        return true;
+      }
+      return false;
     }
 
     @Override