[X86] Improve min/max reduction costs.

This is similar to what I recently did for getArithmeticReductionCost.

I'm trying to account for the narrowing from 512->256->128 as we go.

I've also added a new helper method getMinMaxCost that tries to
handle the cases where we have native min/max instructions and
fall back to cmp+select when we don't.

Differential Revision: https://reviews.llvm.org/D76634