Simplify failedLow away official-stockfish#1986

FailedLow doesn't seem to add any value so remove it.

STC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 43915 W: 9682 L: 9604 D: 24629
http://tests.stockfishchess.org/tests/view/5c5339770ebc592fc7baef74

LTC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 58515 W: 9670 L: 9609 D: 39236
http://tests.stockfishchess.org/tests/view/5c53cc840ebc592fc7baf6c1

Ideas for further work:

    Tune the values in the revised fallingEval calculation
    Consider adding a term using delta, e.g. c * (delta - 20) as an indicator of eval instability

Bench: 3318033

src/search.cpp

@@ -297,7 +297,6 @@ void Thread::search() {
   MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
   double timeReduction = 1.0;
   Color us = rootPos.side_to_move();
-  bool failedLow;
 
   std::memset(ss-7, 0, 10 * sizeof(Stack));
   for (int i = 7; i > 0; i--)
@@ -308,7 +307,7 @@ void Thread::search() {
   beta = VALUE_INFINITE;
 
   if (mainThread)
-      mainThread->bestMoveChanges = 0, failedLow = false;
+      mainThread->bestMoveChanges = 0;
 
   size_t multiPV = Options["MultiPV"];
   Skill skill(Options["Skill Level"]);
@@ -349,7 +348,7 @@ void Thread::search() {
 
       // Age out PV variability metric
       if (mainThread)
-          mainThread->bestMoveChanges *= 0.517, failedLow = false;
+          mainThread->bestMoveChanges *= 0.517;
 
       // Save the last iteration's scores before first PV line is searched and
       // all the move scores except the (new) PV are set to -VALUE_INFINITE.
@@ -429,7 +428,6 @@ void Thread::search() {
                   if (mainThread)
                   {
                       failedHighCnt = 0;
-                      failedLow = true;
                       mainThread->stopOnPonderhit = false;
                   }
               }
@@ -481,19 +479,19 @@ void Thread::search() {
           && !Threads.stop
           && !mainThread->stopOnPonderhit)
       {
-          double fallingEval = (306 + 119 * failedLow + 6 * (mainThread->previousScore - bestValue)) / 581.0;
+          double fallingEval = (306 + 9 * (mainThread->previousScore - bestValue)) / 581.0;
           fallingEval        = std::max(0.5, std::min(1.5, fallingEval));
 
           // If the bestMove is stable over several iterations, reduce time accordingly
           timeReduction = lastBestMoveDepth + 10 * ONE_PLY < completedDepth ? 1.95 : 1.0;
+          double reduction = std::pow(mainThread->previousTimeReduction, 0.528) / timeReduction;
 
           // Use part of the gained time from a previous stable move for the current move
           double bestMoveInstability = 1.0 + mainThread->bestMoveChanges;
-          bestMoveInstability *= std::pow(mainThread->previousTimeReduction, 0.528) / timeReduction;
 
           // Stop the search if we have only one legal move, or if available time elapsed
           if (   rootMoves.size() == 1
-              || Time.elapsed() > Time.optimum() * bestMoveInstability * fallingEval)
+              || Time.elapsed() > Time.optimum() * fallingEval * reduction * bestMoveInstability)
           {
               // If we are allowed to ponder do not stop the search now but
               // keep pondering until the GUI sends "ponderhit" or "stop".