change flat magic pen to magic resistance reduction

magic resistance reduction can take a target's resistance below 0 (abyssal's scepter, amumu auto attack). Flat magic pen should be able to do the same. You would probably have to tune the numbers, however.

What this does is it makes flat magic pen much stronger versus squishy targets. As it should be. But (if you've tuned down the numbers a bit), it makes flat magic pen weaker versus those who have stacked magic resistance. This would further emphasize that buying flat magic pen is a commitment to damage squishies.