Isolated pawns tweak

Give opposed doubled isolated pawns only the Doubled penalty.
The other isolated pawns get the Isolated penalty and the WeakUnopposed penalty.
The popcount condition has been replaced with an opposed check,
which is non-functional, but probably gives a speed-up.

Passed STC (https://tests.stockfishchess.org/tests/view/5ed0f0f0042fa6d77c3552f5):
LLR: 2.96 (-2.94,2.94) {-0.50,1.50}
Total: 121832 W: 23562 L: 23195 D: 75075
Ptnml(0-2): 2092, 14064, 28313, 14279, 2168

LTC: (https://tests.stockfishchess.org/tests/view/5ed22e40042fa6d77c355387)
LLR: 2.96 (-2.94,2.94) {0.25,1.75}
Total: 103368 W: 13232 L: 12768 D: 77368
Ptnml(0-2): 693, 9484, 30919, 9842, 746

closes https://github.com/official-stockfish/Stockfish/pull/2706

Bench: 4085694

src/pawns.cpp

@@ -145,13 +145,13 @@ namespace {
 
         else if (!neighbours)
         {
-            score -=   Isolated
+            if (     opposed
-                     + WeakUnopposed * !opposed;
+                &&  (ourPawns & forward_file_bb(Them, s))
-
-            if (   (ourPawns & forward_file_bb(Them, s))
-                && popcount(opposed) == 1
                 && !(theirPawns & adjacent_files_bb(s)))
                 score -= Doubled;
+            else
+                score -=   Isolated
+                         + WeakUnopposed * !opposed;
         }
 
         else if (backward)