[llvm] [RISCV] Add software pipeliner support (PR #117546)

[Ryotaro Kasuga via llvm-commits]

kasuga-fj wrote:

> Is this PR right? It refers to a PR of AMDGPU target. :-)

Sorry, I made a mistake. #79589 is correct one.

> Well done! I will have a look later!
Currently, we don't have much experiences on pipeliner, so thanks for leading me to your fantastic works, I will follow that!

Thanks for your interest! We are not original committers of `MachinePipeliner`, so there are things we also don't fully understand. It would be very helpful for us if you would contribute to the development together!

https://github.com/llvm/llvm-project/pull/117546