[llvm] [VPlan] Support plain CFG plans in the verifier. (PR #181817)

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

LGTM to land. It turns out we get those same verification failures on main today. But it turns out they only log to stderr and don't actually assert. Which explains why the expensive-checks bots didn't fail. 

https://github.com/llvm/llvm-project/pull/181817