Add functions to check for decisive scores

Thanks to peregrineshahin and robbyrobbyrob for their suggestions.

closes https://github.com/official-stockfish/Stockfish/pull/5696

No functional change

src/types.h

@@ -155,6 +155,21 @@ constexpr Value VALUE_TB                 = VALUE_MATE_IN_MAX_PLY - 1;
 constexpr Value VALUE_TB_WIN_IN_MAX_PLY  = VALUE_TB - MAX_PLY;
 constexpr Value VALUE_TB_LOSS_IN_MAX_PLY = -VALUE_TB_WIN_IN_MAX_PLY;
 
+
+constexpr bool is_valid(Value value) { return value != VALUE_NONE; }
+
+constexpr bool is_win(Value value) {
+    assert(is_valid(value));
+    return value >= VALUE_TB_WIN_IN_MAX_PLY;
+}
+
+constexpr bool is_loss(Value value) {
+    assert(is_valid(value));
+    return value <= VALUE_TB_LOSS_IN_MAX_PLY;
+}
+
+constexpr bool is_decisive(Value value) { return is_win(value) || is_loss(value); }
+
 // In the code, we make the assumption that these values
 // are such that non_pawn_material() can be used to uniquely
 // identify the material on the board.