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

[Pengcheng Wang via llvm-commits]

wangpc-pp wrote:

> 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`

Currently, I have no idea. I was thinking about adapting the scheduling model.

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