[llvm] [LV] Remove selectUserVF, remove use of legacy expectedCost (NFCI) (PR #191216)

[via llvm-commits]

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@@ -5011,43 +4996,26 @@ InstructionCost LoopVectorizationCostModel::expectedCost(ElementCount VF) {
     // For each instruction in the old loop.
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ayalz wrote:

```suggestion
```
goes w/o saying

https://github.com/llvm/llvm-project/pull/191216