Remember when TT value equals static evaluation value

Remember when TT value equals static evaluation value

When the stored TT value equals the static value set a
proper flag so to not call evaluation() if we hit the
same position again but use the stored TT value instead.

This is another trick to avoid calling costly evaluation()
in qsearch.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>