Don't take the split lock if we don't have
available slaves (about 30-40% of times).
This new condition allows to retire the now
redundant one on number of threads.
No functional change.
// Step 19. Check for splitting the search
if ( !SpNode
- && Threads.size() >= 2
&& depth >= Threads.minimumSplitDepth
+ && Threads.available_slave(thisThread)
&& ( !thisThread->activeSplitPoint
|| !thisThread->activeSplitPoint->allSlavesSearching)
&& thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)