[llvm] [RISCV] Factor out common SiFive7 scheduling model into an abstraction layer (PR #144442)

[Pengcheng Wang via llvm-commits]

https://github.com/wangpc-pp approved this pull request.

It is still large but I tried to scan all the changes, more eyes are needed.
LGTM.

https://github.com/llvm/llvm-project/pull/144442