[llvm] [RISCV] Add scheduling information for SiFive VCIX (PR #86093)

[Wang Pengcheng via llvm-commits]

wangpc-pp wrote:

Thanks for doing this! The PR is OK over all I think.
One concern is that: I prefer to restrict vendors' scheduling infos to their own RISCVSchedXXX.td via mechanisms like `InstRW`, `SchedVariant`, etc. Or changes about vendor extensions scheduling will get their hands on other existed scheduling models in tree and downstreams.

https://github.com/llvm/llvm-project/pull/86093