[llvm] [VPlan] Update VPInst::onlyFirstLaneUsed to check users. (PR #80269)

[via llvm-commits]

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

LGTM, adding various comments.

The only observable changes to existing tests involve cse'ing IV steps from replicate regions when unrolling w/o vectorizing, iinm. Is there some other scenario worth testing?

"... this also enables using an Add VPInstruction to model the increment" - as a follow-up?

https://github.com/llvm/llvm-project/pull/80269