Add test for fix to tablegen for custom folders for ops that return a single
variadic result.

Add missing test for single line fix to `void OpEmitter::genFolderDecls()`
entitled "Fold away reduction over 0 dimensions."

PiperOrigin-RevId: 273880337
diff --git a/mlir/test/Transforms/test-canonicalize.mlir b/mlir/test/Transforms/test-canonicalize.mlir
index 7bae125..fdbfd59 100644
--- a/mlir/test/Transforms/test-canonicalize.mlir
+++ b/mlir/test/Transforms/test-canonicalize.mlir
@@ -27,3 +27,11 @@
   }) : (i32) -> (i32)
   return %0 : i32
 }
+
+// CHECK-LABEL: func @remove_op_with_variadic_results_and_folder
+// CHECK-SAME: (%[[ARG_0:[a-z0-9]*]]: i32, %[[ARG_1:[a-z0-9]*]]: i32)
+func @remove_op_with_variadic_results_and_folder(%arg0 : i32, %arg1 : i32) -> (i32, i32) {
+  // CHECK-NEXT: return %[[ARG_0]], %[[ARG_1]]
+  %0, %1 = "test.op_with_variadic_results_and_folder"(%arg0, %arg1) : (i32, i32) -> (i32, i32)
+  return %0, %1 : i32, i32
+}