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

[Craig Topper via llvm-commits]

topperc wrote:

What's the history of why the cost model tests use `-riscv-v-fixed-length-vector-lmul-max=1`? That prevents fixed vector tests from covering LMULs larger than 1.

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