[RISCV][CostModel] Add getRISCVInstructionCost() to TTI for CostKind (#76793)

Instruction cost for CodeSize and Latency/RecipThroughput can be very
different. Considering the diversity of CostKind and vendor-specific
cost, and how they are spread across various TTI functions, it's
becoming quite a challenge to handle. This patch adds an interface
getRISCVInstructionCost to address it.