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

[Florian Hahn via llvm-commits]

================
@@ -530,8 +543,7 @@ bool VPInstruction::onlyFirstLaneUsed(const VPValue *Op) const {
   case VPInstruction::CalculateTripCountMinusVF:
   case VPInstruction::CanonicalIVIncrementForPart:
   case VPInstruction::BranchOnCount:
-    // TODO: Cover additional operands.
-    return getOperand(0) == Op;
+    return true;
----------------
fhahn wrote:

Left as is for now, as this only is needed for this patch at the moment.

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