[llvm] [RISCV][CostModel] Add getCFInstrCost RISC-V implementation (PR #65599)

[Michael Maitland via llvm-commits]

https://github.com/michaelmaitland approved this pull request.

LGTM, but please wait for sign off from @preames since he was the one that asked about performance impact and hasn't had an opportunity to weigh in yet.

https://github.com/llvm/llvm-project/pull/65599