[llvm] [RISCV][TTI] Improve SiFive7 reduction cost (PR #90951)

[Kolya Panchenko via llvm-commits]

nikolaypanchenko wrote:

> I'm not opposed to add processor-specific cost models, but I think we should well organize the code or it may be hard to mantain as time goes on.

Do you have any suggestion? The only other option I can think about is each vendor will provide its own TTI, say `class VendorRISCVTTIImpl : public RISCVTTImpl`

https://github.com/llvm/llvm-project/pull/90951