[llvm] [VPlan] First step towards VPlan cost modeling. (PR #92555)

[Florian Hahn via llvm-commits]

fhahn wrote:

> Given the assertion whack-a-mole, should this change be reverted until after LLVM 19 has branched early next week? (Or maybe just disable the consistency assertion?)

How about an option to enable the legacy behavior, off in `main` and enabled on the release branch? (https://github.com/llvm/llvm-project/pull/99536)

https://github.com/llvm/llvm-project/pull/92555