[llvm] [RISCV][TTI] Support fdiv/udiv/sdiv/srem/urem in getArithmeticInstrCost (PR #89170)

[Shih-Po Hung via llvm-commits]

arcbbb 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.

I create #89886 to help review the removal of `-riscv-v-fixed-length-vector-lmul-max=1`.

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