Do not allow horde first-rank pawns to be doubled

STC
LLR: 2.99 (-2.94,2.94) [-10.00,5.00]
Total: 2274 W: 1149 L: 1097 D: 28
http://35.161.250.236:6543/tests/view/5a730db56e23db0fbab0d346

LTC
LLR: 2.95 (-2.94,2.94) [-10.00,5.00]
Total: 4801 W: 2387 L: 2354 D: 60
http://35.161.250.236:6543/tests/view/5a735a4c6e23db0fbab0d349

src/pawns.cpp

@@ -561,6 +561,11 @@ namespace {
         stoppers   = theirPawns & passed_pawn_mask(Us, s);
         lever      = theirPawns & PawnAttacks[Us][s];
         leverPush  = theirPawns & PawnAttacks[Us][s + Up];
+#ifdef HORDE
+        if (pos.is_horde() && relative_rank(Us, s) == RANK_1)
+            doubled = 0;
+        else
+#endif
         doubled    = ourPawns   & (s - Up);
         neighbours = ourPawns   & adjacent_files_bb(f);
         phalanx    = neighbours & rank_bb(s);