Use Movepick SEE value in search

This halves the calls to the costly pos.see_sign(),
speed up is about 1-1.3%

Non functional change.

src/movepick.cpp

@@ -243,6 +243,11 @@ void MovePicker::generate_next_stage() {
   }
 }
 
+int MovePicker::see_sign() const
+{
+  return  stage == GOOD_CAPTURES ?  1
+        : stage == BAD_CAPTURES  ? -1 : 0;
+}
 
 /// next_move() is the most important method of the MovePicker class. It returns
 /// a new pseudo legal move every time it is called, until there are no more moves