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

[via llvm-commits]

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@@ -512,7 +518,14 @@ 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 {
+      assert((getOpcode() == VPInstruction::ComputeReductionResult ||
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ayalz wrote:

Ahh, right.
This in particular would be more clearly expressed by doing
`State.set(this, GeneratedValue, Part, true /* scalar */);`
rather than explicitly storing the single **last** lane directly in the **first** lane zero via
`State.set(this, GeneratedValue, VPIteration(Part, 0));`

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