[RISCV] Key VectorIntrinsicCostTable by SEW [nfc-ish]

Previously, we'd keyed the table by the vector type, but we were actually assigning the same cost for all the types with a common element type.  Unless we'd missed an entry, this means that effectively we were performing an SEW lookup.

Restructure the table to make this SEW dependence more explicit, and in the process greatly reduce the size of the table.