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

[Philip Reames via llvm-commits]

https://github.com/preames commented:

Do we really have no cost-model testing for any of the places you're changing?  If so, you should really fix that before landing this change.

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