[llvm] [LegalizeTypes] Use vector CTPOP for wide integer popcount on VPOPCNTDQ targets (PR #182675)

[Xavier Roche via llvm-commits]

xroche wrote:

Thanks for the review and the pointer to `ReplaceNodeResults` — that makes a lot more sense as the right hook for this, keeping it X86-specific rather than in generic legalization.

I'll close this PR. You're clearly the most knowledgeable person on this part of the backend, and I suspect you'd produce a much better implementation than we would. That said, if it's helpful and doesn't create too much noise, we're happy to take a stab at reworking it in `ReplaceNodeResults` — just let us know.

Thanks also for pulling out the KNM test coverage into #182676, that's appreciated.

(Reminder: this contribution was LLM-assisted.)

https://github.com/llvm/llvm-project/pull/182675