[X86] canLowerByDroppingEvenElements - generalize to drop even or odd elements

This allows us to match shuffle<1,3,5,7,9,11,13,15> style shift+trunc/pack patterns as well as the existing shuffle<0,2,4,6,8,10,12,14> style shuffle trunc/pack patterns

In the future, interleaving patterns might benefit from an even more general implementation for higher strides