deal with the scope of a court's authority to exclude evidence that "[r]easonable jurists could conclude ... was relevant mitigating evidence." Tennard, 542 U.S. at 288, 124 S.Ct. 2562; McKoy, 494 U.S. at 442, 110 S.Ct. 1227 ("Under our decisions, it is not relevant whether the bar- rier to the s