[llvm] [RISCV][MI] Support partial spill/reload for vector registers (PR #105661)

[Brandon Wu via llvm-commits]

4vtomat wrote:

> Just an idea, but in #107446 there are subregisters added for GPR which reduces the spill size. Can we do the same for VR and add subregisters for fractional LMULs? Would that help with the spills?

I also agree with this approach since it's more consistent and correct way to resolve the underlying problem of modeling fractional VRegs. I'm not sure why we didn't consider that approach at the first time, instead using `TSFlags` to encode fractional VReg information, do you think we should do that ultimately?

https://github.com/llvm/llvm-project/pull/105661