[llvm] [VPlan] Model entry-mask of VPBB in VPlan (NFC) (PR #165862)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> Perhaps I'm missing something, but I don't really understand the goal of this PR. Is this refactoring code in preparation for future work? Or is it attempting to reduce compile time, improve readability, etc?

The goal of this PR is to make sure that things that can be modeled directly in VPlan aren't modeled using some side-channel: I don't have any follow-ups planned at the moment, so I think it is currently a nice cleanup. 

https://github.com/llvm/llvm-project/pull/165862