[llvm] [VPlan] Return Max from computeCost without underlying instrs. (PR #109708)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

The latest version changed quite drastically. 

Now no sentinel value is used. Instead VPSingleDefRecipe/VPInterleaveRecipe return their cost via legacy, which removes the use of `getInstructionForCost` to return legacy costs. `getInstructionForCost` is only used in ::cost to divide what to skip and when to apply forced costs.

Hopefully this should be simpler and clearer. WDYT?

https://github.com/llvm/llvm-project/pull/109708