[llvm] [RISCV] Fix missing scaling by LMUL in cost model (PR #73342)

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

Thanks, the changes in the cost model tests look good to me. Just one minor thing in the SLP test, otherwise LGTM

https://github.com/llvm/llvm-project/pull/73342