[llvm] [RISCV][GISel] Rename XLenVT sXLen to be consistent with other LLTs. (PR #70288)

[Min-Yih Hsu via llvm-commits]

https://github.com/mshockwave approved this pull request.

Personally I like LLT to be in the name since I sometimes got confused. But I think consistency is more important in a long term, thus LGTM

https://github.com/llvm/llvm-project/pull/70288