[llvm] [DAG] Fold bitreverse(shl/srl(bitreverse(x),y)) -> srl/shl(x,y) (PR #89897)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

> Can you also do the globalisel equivalent?

Sure, I haven't worked with combinerhelper much, but I'm assuming it shouldn't be that tricky.

https://github.com/llvm/llvm-project/pull/89897