[llvm] [RISCV] Use VP's interfaces to reconstruct cast/cmp/binop cost. NFC (PR #115978)

[Luke Lau via llvm-commits]

lukel97 wrote:

Hi, I noticed we have very similar code for this in BasicTTIImpl.h so I opened up an alternative PR that would deduplicate these: https://github.com/llvm/llvm-project/pull/115983

https://github.com/llvm/llvm-project/pull/115978