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

[Shih-Po Hung via llvm-commits]

arcbbb wrote:

> I just wonder if we can make cost model bind to scheduling model.

Considering
1. The interpretation of Latency and ReleaseAtCycles in the scheduling model may not align directly with TCK_Latency and TCK_RecipThroughput in the TTI cost model.
2. In the scheduling model, the vector length is assumed to be VLMAX, whereas in the TTI cost model, we can derive the element count from fixed-length vector types.

It might be better to keep them separate for now.

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