[llvm] [RISCV] Generalize cost model for vp_fneg. NFC. (PR #126915)

[Mikhail R. Gadelha via llvm-commits]

mikhailramalho wrote:

I didn't add new tests since we already have tests for it on `llvm/test/Analysis/CostModel/RISCV/arith-fp.ll`

https://github.com/llvm/llvm-project/pull/126915