[RISCV][NFC]: Refactor classes for load/store instructions of RVV

This patch refactors classes for load/store of V extension by:
- Introduce new class for VUnitStrideLoadFF and VUnitStrideSegmentLoadFF
  so that uses of L/SUMOP* are not spread around different places.
- Reorder classes for Unit-Stride load/store in line with table
  describing lumop/sumop in riscv-v-spec.pdf.

Reviewed By: HsiangKai, craig.topper

Differential Revision: https://reviews.llvm.org/D109318