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

[Florian Hahn via llvm-commits]

fhahn wrote:

> > As per [#168738 (comment)](https://github.com/llvm/llvm-project/pull/168738#discussion_r2549411124), this fixes an mis-compile when vectorizing early-exit loops + interleaving.
> 
> Given the best ZeroIsPoison value is dependent on the type of vectorisation, is it possible to set it based on this information rather than dropping to the safest value?

FirstActiveLane is currently only used in this mode AFAIK.

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