Delay killers[] initialization

Most of the time we cut-off earlier, at captures, so this
results in useless work.

There is a small functionality change becuase 'ss' can change
from MovePicker c'tor to when killers are tried due, for
instance, to singular search.

bench: 4603795

src/movepick.h

@@ -97,6 +97,7 @@ private:
   const Position& pos;
   const HistoryStats& history;
   Search::Stack* ss;
+  Move* countermoves;
   Depth depth;
   Move ttMove;
   MoveStack killers[4];