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

[via llvm-commits]

LiqinWeng 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: #115983

OK~

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