[llvm] [UTC][VPlan] Use `-vplan-print-after` for VPlan-dump-based tests (PR #178736)

[Florian Hahn via llvm-commits]

fhahn wrote:

> > Please let me know if you'd like me to merge this once the merge conflicts are fixed
> 
> I thought I replied earlier but I don't see it...
> 
> Anyway, yes, please. Conflicts have been resolved.

Sure, could you add a bit more details to the PR description?

https://github.com/llvm/llvm-project/pull/178736