[llvm] [VPlan] Introduce VPlanConstantFolder (PR #125365)

[Ramkumar Ramachandra via llvm-commits]

https://github.com/artagnon commented:

> Hmm, I am a bit surprised there's not more to fold in the existing test cases. Any chance we could at least one test covering each fold?

I was actually very surprised when I authored the patch and saw so few test changes. Yes, will write tests to cover each fold, along with the other pending items in this review.

https://github.com/llvm/llvm-project/pull/125365