[llvm] [RISCV][TTI] Refine the cost of FCmp (PR #88833)

[Shih-Po Hung via llvm-commits]

arcbbb wrote:

> Can you separate the "Support f16 operations using zvfhmin in the absence of zvfh" piece into it's own review? The rest of this looks pretty straight forward, and once that bit is pulled out, probably an easy LGTM

Sure. will create a separate pr after this.

https://github.com/llvm/llvm-project/pull/88833