[RISCV] Refine cost on Min/Max reduction (#79402)

This patch is split off from #77342, and follows #79103

- Correct for CodeSize cost that 1 instruction is not included. 3 is
from {VMV.S, ReductionOp, VMV.X}
- Add SplitCost which chains a series of VMAX/VMIN/... which scales with
LMUL.
- Use MVT to estimate VL.