[llvm] [VPlan] Handle FirstActiveLane when unrolling. (PR #145394)

[via llvm-commits]

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@@ -856,9 +856,32 @@ Value *VPInstruction::generate(VPTransformState &State) {
     return Builder.CreateOrReduce(Res);
   }
   case VPInstruction::FirstActiveLane: {
-    Value *Mask = State.get(getOperand(0));
-    return Builder.CreateCountTrailingZeroElems(Builder.getInt64Ty(), Mask,
-                                                true, Name);
+    if (getNumOperands() == 1) {
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ayalz wrote:

```suggestion
    auto Int64 = Builder.getInt64Ty();
    if (getNumOperands() == 1) {
```
to be reused in multiple cases below.

https://github.com/llvm/llvm-project/pull/145394