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

[Florian Hahn via cfe-commits]

fhahn wrote:

> 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?

I wasn't able to come up with a different test with vectorizing.

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