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

[via llvm-commits]

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@@ -788,13 +793,13 @@ void VPlan::prepareToExecute(Value *TripCountV, Value *VectorTripCountV,
   }
 
   for (unsigned Part = 0, UF = State.UF; Part < UF; ++Part)
-    State.set(&VectorTripCount, VectorTripCountV, Part);
+    State.set(&VectorTripCount, VectorTripCountV, VPIteration(Part, 0));
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ayalz wrote:

Independent of this patch: would be good to be consistent if/when a scalar that is  invariant or uniform-across-parts is set in lane zero of all parts (e.g., VectorTripCount) or in lane zero of part zero only (e.g., VFxUF).

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