Replace scanMonitorLookupCaches() loop with memset #24803

scanMonitorLookupCaches() iterates over each slot in J9VMThread->objectMonitorLookupCache and calls the virtual method doMonitorLookupCacheSlot() to zero it during a GC root scan. This uses the pattern of looping over the cache slots and dispatching a virtual method to perform the intended GC specific operation.

This pattern is not utilized for this use-case, and the cache slots are only zeroed for all GCs. Hence the loop and if-condition are unnecessary and can be replaced with some memset.

scanMonitorLookupCaches():
...
    for (; cacheIndex < J9VMTHREAD_OBJECT_MONITOR_CACHE_SIZE; cacheIndex++) {
    	doMonitorLookupCacheSlot(&objectMonitorLookupCache[cacheIndex]);     
    }
...
 
doMonitorLookupCacheSlot():
    if (0 != *slotPtr) {
        *slotPtr = 0;   // just zero it out
    }

fyi: @amicic