[llvm] [RISCV][CostModel][NFC] Add getRISCVInstructionCost() to TTI for Cost… (PR #73651)

[Wang Pengcheng via llvm-commits]

https://github.com/wangpc-pp commented:

I think this makes how the cost is calculated much more consistent and clearer.

https://github.com/llvm/llvm-project/pull/73651