[VPlan] Add VPWidenCastRecipe, split off from VPWidenRecipe (NFCI).

To generate cast instructions, the result type is needed. To allow
creating widened casts without underlying instruction, introduce a new
VPWidenCastRecipe that also holds the result type.

This functionality will be used in a follow-up patch to
implement truncateToMinimalBitwidths as VPlan-to-VPlan transform.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D149081