[RISCV] Support STEP_VECTOR with a step greater than one

DAGCombiner was recently taught how to combine STEP_VECTOR nodes,
meaning the step value is no longer guaranteed to be one by the time it
reaches the backend for lowering.

This patch supports such cases on RISC-V by lowering to other step
values to a multiply following the vid.v instruction. It includes a
small optimization for common cases where the multiply can be expressed
as a shift left.