[llvm] [RISCV] Add DAG combine to turn (sub (shl X, 8-Y), (shr X, Y)) into orc.b (PR #111828)

[Nemanja Ivanovic via llvm-commits]

nemanjai wrote:

> > (sub (shl X, 8 - Y), (srl X, Y)) => (orc.b X)
> 
> Can you provide alive2 proofs?

Do you have a link to a guide for how to use alive2 for proofs where the target of the optimization is a RISC-V instruction?

https://github.com/llvm/llvm-project/pull/111828