[CostModel] Treat 'widen subvector' patterns as zero cost

As discussed on D107228, widening a subvector by inserting the whole subvector into the bottom a larger undef vector should always be cheap enough that we can treat it as zero cost.

NOTE: If this proves to cause issues we have the option of introducing a "SK_WidenSubvector" shuffle kind enum that targets could override the zero cost, but that doesn't seem necessary atm.

Differential Revision: https://reviews.llvm.org/D107228