Simplify IID depth formula

Restore original formula messed up during
half-ply removal.

STC
LLR: 4.11 (-2.94,2.94) [-3.00,1.00]
Total: 21349 W: 4091 L: 3909 D: 13349

LTC
LLR: 5.42 (-2.94,2.94) [-3.00,1.00]
Total: 52819 W: 8321 L: 8122 D: 36376

bench: 8040572

src/search.cpp

@@ -769,9 +769,9 @@ namespace {
  && !ttMove
  && (PvNode || ss->staticEval + 256 >= beta))
  {
- Depth d = 2 * (depth - 2 * ONE_PLY) - (PvNode ? DEPTH_ZERO : depth / 2);
+ Depth d = depth - 2 * ONE_PLY - (PvNode ? DEPTH_ZERO : depth / 4);
  ss->skipEarlyPruning = true;
- search<PvNode ? PV : NonPV, false>(pos, ss, alpha, beta, d / 2, true);
+ search<PvNode ? PV : NonPV, false>(pos, ss, alpha, beta, d, true);
  ss->skipEarlyPruning = false;
 
  tte = TT.probe(posKey, ttHit);
@@ -821,13 +821,11 @@ namespace {
  if (!pos.legal(move, ci.pinned))
  continue;
 
- moveCount = ++splitPoint->moveCount;
+ ss->moveCount = moveCount = ++splitPoint->moveCount;
  splitPoint->spinlock.release();
  }
  else
- ++moveCount;
-
- ss->moveCount = moveCount;
+ ss->moveCount = ++moveCount;
 
  if (RootNode)
  {
@@ -935,8 +933,7 @@ namespace {
  // Check for legality just before making the move
  if (!RootNode && !SpNode && !pos.legal(move, ci.pinned))
  {
- moveCount--;
- ss->moveCount = moveCount;
+ ss->moveCount = --moveCount;
  continue;
  }
 
@@ -1429,7 +1426,7 @@ namespace {
  }
 
  // Extra penalty for PV move in previous ply when it gets refuted
- if (is_ok((ss-2)->currentMove) && (ss-1)->moveCount==1 && !pos.captured_piece_type())
+ if (is_ok((ss-2)->currentMove) && (ss-1)->moveCount == 1 && !pos.captured_piece_type())
  {
  Square prevPrevSq = to_sq((ss-2)->currentMove);
  HistoryStats& ttMoveCmh = CounterMovesHistory[pos.piece_on(prevPrevSq)][prevPrevSq];