[llvm] [RISCV] Combine vslideup_vl with known VL to a smaller LMUL (PR #66671)

[Luke Lau via llvm-commits]

lukel97 wrote:

Marking this as a draft as it needs rebased after #66087 and #65997 have landed. It also remains to be seen if the combine approach will be able to fully subsume performing the reduction in lowering.

https://github.com/llvm/llvm-project/pull/66671