[llvm] [RISCV][TTI] Fix shift VV opcode mapping in getArithmeticInstrCost (PR #156408)

[Luke Lau via llvm-commits]

https://github.com/lukel97 requested changes to this pull request.

This won't have any difference because the RISCV opcodes all have the same cost and are generally expected to. There's not really any benefit here to making it more granular

https://github.com/llvm/llvm-project/pull/156408