[llvm] [VPlan] Consistently use (Part, 0) for first lane scalar values (PR #80271)

[via llvm-commits]

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@@ -512,7 +513,16 @@ void VPInstruction::execute(VPTransformState &State) {
     if (!hasResult())
       continue;
     assert(GeneratedValue && "generateInstruction must produce a value");
-    State.set(this, GeneratedValue, Part);
+    if (GeneratedValue->getType()->isVectorTy()) {
+      State.set(this, GeneratedValue, Part);
+    } else {
+      // Note that ComputeReductionResult creates a scalar, but has its last
+      // lane accessed, via the generic  VPLiveOut.
+      assert((getOpcode() == VPInstruction::ComputeReductionResult ||
+              State.VF.isScalar() || vputils::onlyFirstLaneUsed(this)) &&
+             "scalar value but not only first lane used");
+      State.set(this, GeneratedValue, VPIteration(Part, 0));
----------------
ayalz wrote:

```suggestion
      State.set(this, GeneratedValue, Part, true /* scalar */);
```

https://github.com/llvm/llvm-project/pull/80271