[llvm] [VPlan] First step towards VPlan cost modeling (LegacyCM in CostCtx) (PR #92555)

[via llvm-commits]

https://github.com/ayalz commented:

This looks right to me, thanks! Adding various comments.
Worth noting in the commit message that the cost of VPWidenRecipe is computed directly, other recipes to follow.

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