[llvm] [RISCV][RFC] Add additional opcodes to RISCVDAGToDAGISel::hasAllNBitUsers (PR #147728)

[Alex Bradbury via llvm-commits]

asb wrote:

Thanks for the note on ORC_B/BREV8. I've incorporated the corrected ORC_B/BREV8 logic from #148076 and the SLLIW improvement from #143844 and updated to HEAD.

What are your thoughts on:
* If there should be any particular threshold for adding things to the ISel hasAllNBitUsers or we should just keep it in sync best effort (as this PR tries to do)
* Testing

https://github.com/llvm/llvm-project/pull/147728