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

[Alex Bradbury via llvm-commits]

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

LGTM, but feel free to wait for review from those who may have developed more emotional attachment to the current naming!

sDoubleXLen seems fine and is probably easier to read than s2Xlen.

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