[llvm] [VPlan] First step towards VPlan cost modeling. (PR #67934)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

I think I forgot to mention this for completeness, but this depends on https://github.com/llvm/llvm-project/pull/89386

> Similar to how code-gen is simplified, modularized and kept consistent by breaking down ILV into VPlan/Region/Block/Recipe::execute() - in a gradual process which still utilizes ILV methods via VPTransformState, could compute-cost be driven by VPlan/Region/Block/Recipe::computeCost - initially utilizing CM methods internally where needed, by passing a CM* in VPCostContext? That should help keep code-gen and its cost aligned and consistent at each scope.

The patch intentionally avoided making CM part of `VPCostContext`, to keep a clear separation between VPlan-based and legacy costs, to avoid leaking information from the legacy cost model and avoid introducing new uses of the legacy cost model at this point.

https://github.com/llvm/llvm-project/pull/67934