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

[David Sherwood 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);
----------------
david-arm wrote:

```
/// Returns true if only the first lane of \p Def is used.
bool onlyFirstLaneUsed(const VPValue *Def);
```

I must admit I find this area of the code a bit difficult to understand. I can see that for icmp we do something like this:

```
  case Instruction::ICmp: {
    bool OnlyFirstLaneUsed = vputils::onlyFirstLaneUsed(this);
    Value *A = State.get(getOperand(0), OnlyFirstLaneUsed);
    Value *B = State.get(getOperand(1), OnlyFirstLaneUsed);
    return Builder.CreateCmp(getPredicate(), A, B, Name);
  }
```

but if I do the same thing for extractelement then the answer to `vputils::onlyFirstLaneUsed(this)` should always be false, right? Since we may use any lane of the vector passed as operand 0.

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