[llvm] [RISCV] Rework IDiv and FDiv pipes on SiFive7 (PR #73970)

[Wang Pengcheng via llvm-commits]

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

LGTM.
(Is this not covered by some llvm-mca tests?)

https://github.com/llvm/llvm-project/pull/73970