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

[via llvm-commits]

https://github.com/ayalz commented:

Very nice step forward!!

Making the last decision, namely, selecting which VPlan has the best cost, based on (partially) VPlan-based cost computation, is a good starting point, gradually allowing earlier cost-based decisions to take place along the VPlan-to-VPlan transformation pipeline.

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.

Adding various comments inline after a first pass.

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