[llvm] [RISCV] Support vp.{gather,scatter} in RISCVGatherScatterLowering (PR #122232)

[Pengcheng Wang via llvm-commits]

https://github.com/wangpc-pp approved this pull request.

Generally LGTM.
The vectorizer way won't be that complicated according to my experiences on porting EPI's implementation, so we may try that first.

https://github.com/llvm/llvm-project/pull/122232