Improve comment

Author: RKSimon

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -6405,8 +6405,9 @@ static SDValue foldCONCAT_VECTORS(const SDLoc &DL, EVT VT,
   if (VT.isScalableVector())
     return SDValue();
 
-  // A CONCAT_VECTOR with all UNDEF/BUILD_VECTOR operands can be
-  // simplified to one big BUILD_VECTOR.
+  // A CONCAT_VECTOR of scalar sources, such as UNDEF, BUILD_VECTOR and
+  // single-element INSERT_VECTOR_ELT operands can be simplified to one big
+  // BUILD_VECTOR.
   // FIXME: Add support for SCALAR_TO_VECTOR as well.
   EVT SVT = VT.getScalarType();
   SmallVector<SDValue, 16> Elts;