Adjust OperationTreeVerifier and TestOperationVisitor to verify new constraints.

src/Test/Utilities/Portable/Compilation/OperationTreeVerifier.cs

@@ -523,6 +523,7 @@ public override void VisitForEachLoop(IForEachLoopOperation operation)
             LogString(nameof(IForEachLoopOperation));
             LogLoopStatementHeader(operation);
 
+            Assert.NotNull(operation.LoopControlVariable);
             Visit(operation.LoopControlVariable, "LoopControlVariable");
             Visit(operation.Collection, "Collection");
             Visit(operation.Body, "Body");

src/Test/Utilities/Portable/Compilation/TestOperationVisitor.cs

@@ -272,22 +272,7 @@ public override void VisitForEachLoop(IForEachLoopOperation operation)
             Assert.Equal(LoopKind.ForEach, operation.LoopKind);
             VisitLocals(operation);
 
-            IEnumerable<IOperation> children;
-
-            // operation.LoopControlVariable shouldn't be null.
-            // The following conditional logic should be removed once https://github.com/dotnet/roslyn/issues/23820
-            // is fixed.
-            if (operation.LoopControlVariable == null)
-            {
-                Assert.Equal(LanguageNames.CSharp, operation.Language);
-                children = new[] { operation.Collection, operation.Body };
-            }
-            else
-            {
-                children = new[] { operation.Collection, operation.LoopControlVariable, operation.Body };
-            }
-
-            children = children.Concat(operation.NextVariables);
+            IEnumerable<IOperation> children = new[] { operation.Collection, operation.LoopControlVariable, operation.Body }.Concat(operation.NextVariables);
             AssertEx.Equal(children, operation.Children);
         }