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

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

> 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?

Currently we are still looking up masks for recipes via their associated block, which means we need to keep around a mapping between masks & blocks, which in turn needs to be kept in sync.

Removing that is great, but I am not sure making VPBasicBlock a VPUser is necessarily the best way forward. Previously masks were part of VPBasicBlock, but they got removed a number of years ago now.

An alternative would be to allow any VPInstruction to be masked (as in the out-of-date https://github.com/llvm/llvm-project/pull/142285, will check what is needed to update it).

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