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

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

> Personally I like LLT to be in the name since I sometimes got confused

Sounds like you are suggesting this change && s16LLT, s32LLT, ...

The `LLT` designation is available to the reader if they look at the type is what made me comfortable giving this a LGTM.

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