[llvm] [LV][NFC] Refactor code for extracting first active element (PR #131118)

[Mel Chen via llvm-commits]

================
@@ -461,6 +461,11 @@ Value *VPInstruction::generate(VPTransformState &State) {
     Value *A = State.get(getOperand(0));
     return Builder.CreateNot(A, Name);
   }
+  case Instruction::ExtractElement: {
+    Value *Vec = State.get(getOperand(0));
+    Value *Idx = State.get(getOperand(1), true);
----------------
Mel-Chen wrote:

Maybe need to add 
```
  case Instruction::ExtractElement:
    return Op == getOperand(1);
```
into `VPInstruction::onlyFirstLaneUsed`.

https://github.com/llvm/llvm-project/pull/131118