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

[Paul Walker via llvm-commits]

paulwalker-arm 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.

Fair enough.

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