Improve AttackEntitiesByPreference by short-circuiting on best possible preference.
Similar trick to D3446 / rP25102 - we can short-circuit if we find units that match our best possible preference.
This is, in particular, almost a 40% wall time improvement on Combat Demo Huge, but it should apply in a number of more normal cases.
Differential Revision: https://code.wildfiregames.com/D5020