[llvm] [RISCV][TTI] Scale the cost of ArithmeticInstr with LMUL (PR #89170)

[Philip Reames via llvm-commits]

https://github.com/preames requested changes to this pull request.

The code you're replacing is already linear in LMUL; your test changes are caused by adding new instructions to the switch.

Please update your review description to match and to justify the change,.  

https://github.com/llvm/llvm-project/pull/89170