[llvm] [RISCV] Add SiFive X390 scheduling model (PR #143938)

[Min-Yih Hsu via llvm-commits]

mshockwave wrote:

> Just a general comment: would the abstraction make the code hard to maintain compared to putting X380 model into a standalone file?

The idea behind a common abstraction is that any update on the scheduling models of X390 and SiFive7 cores will only need to be done once, as they share lots common performance characteristics. So it actually lowers the maintenance cost

https://github.com/llvm/llvm-project/pull/143938