Update comment

Author: nicolasvasilache

mlir/lib/Dialect/Vector/IR/VectorOps.cpp

@@ -4697,7 +4697,8 @@ struct TransferReadAfterWriteToBroadcast
     if (!bcast)
       return failure();
     // At this point, we know we have a bcast.
-    // The masked case is too complext atm, bail.
+    // Bail in the masked case (too complext atm and needed to properly account
+    // for padding).
     if (readOp.getMask() || defWrite.getMask())
       return failure();
     // If indices are not the same a shift may be required, bail.