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

[via llvm-commits]

================
@@ -1256,23 +1256,7 @@ class VPInstruction : public VPRecipeWithIRFlags {
     }
   }
 
----------------
ayalz wrote:

nit: leave the documentation line here in .h even when the implementation moves to .cpp?

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