[llvm] [VPlan] Return invalid cost if any skeleton block has invalid costs. (PR #151940)

[via llvm-commits]

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@@ -1072,9 +1072,13 @@ InstructionCost VPlan::cost(ElementCount VF, VPCostContext &Ctx) {
   // blocks, like the preheader or middle blocks.
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ayalz wrote:

 Comment deserves updating?

https://github.com/llvm/llvm-project/pull/151940