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

[Min-Yih Hsu via llvm-commits]

mshockwave wrote:

> LGTM. I still feel uncomfortable about the complexity (too many conditions, and I personally prefer straightforward definitions) but I think we can improve it progressively.

Yeah I agree the complexity is a bit daunting. I feel like fundamentally the scheduling model infrastructure should be more ergonomic on creating hierarchical scheduling models. In fact, many many moons ago on the inception of the current instr scheduling model, I think people _were_ thinking about making a way to have a "base" / parent scheduling model (there is a comment left somewhere in the TableGen code that shows this) but obviously it went nowhere. 

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