[llvm] [VPlan] Set ZeroIsPoison=false for FirstActiveLane (PR #169298)

[Florian Hahn via llvm-commits]

fhahn wrote:

(There will be a new user after https://github.com/llvm/llvm-project/pull/149042 re-lands, which also requires `ZeroIsPoison=false` as default)

https://github.com/llvm/llvm-project/pull/169298