[llvm] [PowerPC] Check value uses in ValueBit tracking (PR #66040)

[Qiu Chaofan via llvm-commits]

ecnelises wrote:

> Since developer explicitly uses __rldimi, what the developer expects should be the corresponding PPC instruction rldimi?

We can definitely create a new intrinsic for `rldimi`. But I'm concerned whether the rest 8 `rl*` instructions also need such an intrinsic? Current codegen tests won't show changes after adding a new intrinsic, so I may need to find whether some C cases will go worse.

Yes, this patch is indeed a workaround. The reason of this suboptimal codegen is local greedy BitPermutation selector. For longer consideration, we need modeling and recombination of rotate-clear/mask/insert operations in backend. If that shows good result, BitPermutation can be dropped.

https://github.com/llvm/llvm-project/pull/66040