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

[Yingwei Zheng via llvm-commits]

https://github.com/dtcxzyw approved this pull request.

LGTM. Thank you!
Please add the generalized alive proof link to PR description.


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