[llvm] [VPlan] Set ZeroIsPoison=false for FirstActiveLane (PR #169298)

[Florian Hahn via llvm-commits]

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@@ -1005,7 +1005,7 @@ Value *VPInstruction::generate(VPTransformState &State) {
     if (getNumOperands() == 1) {
       Value *Mask = State.get(getOperand(0));
       return Builder.CreateCountTrailingZeroElems(Builder.getInt64Ty(), Mask,
-                                                  true, Name);
+                                                  /*ZeroIsPoison=*/false, Name);
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fhahn wrote:

Could you extend the documentation for `FirstActiveLane` to state explicitly that any of the operands can be all zeros?

https://github.com/llvm/llvm-project/pull/169298