[llvm] [LV] Return Invalid from getLegacyCost when instruction cost forced. (PR #154543)

[David Sherwood via llvm-commits]

================
@@ -6760,9 +6760,10 @@ void LoopVectorizationPlanner::plan(ElementCount UserVF, unsigned UserIC) {
 
 InstructionCost VPCostContext::getLegacyCost(Instruction *UI,
                                              ElementCount VF) const {
-  if (ForceTargetInstructionCost.getNumOccurrences())
+  InstructionCost Cost = CM.getInstructionCost(UI, VF);
+  if (Cost.isValid() && ForceTargetInstructionCost.getNumOccurrences())
     return InstructionCost(ForceTargetInstructionCost.getNumOccurrences());
-  return CM.getInstructionCost(UI, VF);
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david-arm wrote:

This is not a fault with your patch, but it looks like it was already broken due to this:

```
  return InstructionCost(ForceTargetInstructionCost.getNumOccurrences());
```

I'm pretty sure it should be:

```
  return InstructionCost(ForceTargetInstructionCost);
```

similar to `LoopVectorizationCostModel::expectedCost`

https://github.com/llvm/llvm-project/pull/154543