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

[via cfe-commits]

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@@ -512,7 +518,17 @@ 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 {
+      if (getOpcode() == VPInstruction::ComputeReductionResult) {
+        State.set(this, GeneratedValue, VPIteration(Part, 0));
+      } else {
+        assert((State.VF.isScalar() || vputils::onlyFirstLaneUsed(this)) &&
+               "scalar value but not only first lane used");
+        State.set(this, GeneratedValue, VPIteration(Part, 0));
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ayalz wrote:

nit: can simplify into a single State.set() with the condition folded into the assert.

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