Compute SplitPoint::spLevel on the fly

And retire a redundant field. This is important also
from a concept point of view becuase we want to keep
SMP structures as simple as possible with the only
strictly necessary data.

Verified with

dbg_hit_on(sp->spLevel != level)

that the values are 100% the same out of more 50K samples.

No functional change.

src/search.cpp

@@ -1604,7 +1604,12 @@ void Thread::idle_loop() {
                       && sp->slavesCount < MAX_SLAVES_PER_SPLITPOINT
                       && available_to(Threads[i]))
                   {
-                      int score = sp->spLevel * 256 * 256 + sp->slavesCount * 256 - sp->depth * 1;
+                      // Compute the recursive split points chain size
+                      int level = -1;
+                      for (SplitPoint* spp = Threads[i]->activeSplitPoint; spp; spp = spp->parentSplitPoint)
+                          level++;
+
+                      int score = level * 256 * 256 + sp->slavesCount * 256 - sp->depth * 1;
 
                       if (score < bestScore)
                       {
@@ -1618,7 +1623,7 @@ void Thread::idle_loop() {
               if (bestSp)
               {
                   sp = bestSp;
- 
+
                   // Recheck the conditions under lock protection
                   Threads.mutex.lock();
                   sp->mutex.lock();

src/thread.cpp

@@ -154,7 +154,6 @@ void Thread::split(Position& pos, Stack* ss, Value alpha, Value beta, Value* bes
 
   sp.masterThread = this;
   sp.parentSplitPoint = activeSplitPoint;
-  sp.spLevel = activeSplitPoint ? activeSplitPoint->spLevel + 1 : 0;
   sp.slavesMask = 0, sp.slavesMask.set(idx);
   sp.slavesCount = 1;
   sp.depth = depth;
@@ -184,7 +183,7 @@ void Thread::split(Position& pos, Stack* ss, Value alpha, Value beta, Value* bes
 
   Thread* slave;
 
-  while (    sp.slavesCount < MAX_SLAVES_PER_SPLITPOINT 
+  while (    sp.slavesCount < MAX_SLAVES_PER_SPLITPOINT
          && (slave = Threads.available_slave(this)) != NULL)
   {
       sp.slavesMask.set(slave->idx);

src/thread.h

@@ -73,7 +73,6 @@ struct SplitPoint {
   const Position* pos;
   Search::Stack* ss;
   Thread* masterThread;
-  int spLevel;
   Depth depth;
   Value beta;
   int nodeType;