!fixup restore comment and assert for onlyFirstLaneUsed

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -2231,7 +2231,10 @@ class VPReductionPHIRecipe : public VPHeaderPHIRecipe,
   /// Returns true, if the phi is part of an in-loop reduction.
   bool isInLoop() const { return IsInLoop; }
 
+  /// Returns true if the recipe only uses the first lane of operand \p Op.
   bool onlyFirstLaneUsed(const VPValue *Op) const override {
+    assert(is_contained(operands(), Op) &&
+    "Op must be an operand of the recipe");
     return isOrdered() || isInLoop();
   }
 };