Fix decompiling iterator method that consists only of "yield break;".

ICSharpCode.Decompiler/CSharp/CSharpDecompiler.cs

@@ -630,9 +630,13 @@ namespace ICSharpCode.Decompiler.CSharp
 
 			AddDefinesForConditionalAttributes(function);
 			var statementBuilder = new StatementBuilder(specializingTypeSystem, decompilationContext, method, function);
-			entityDecl.AddChild(statementBuilder.ConvertAsBlock(function.Body), Roles.Body);
+			var body = statementBuilder.ConvertAsBlock(function.Body);
+			entityDecl.AddChild(body, Roles.Body);
 
 			if (function.IsIterator) {
+				if (!body.Descendants.Any(d => d is YieldReturnStatement || d is YieldBreakStatement)) {
+					body.Add(new YieldBreakStatement());
+				}
 				RemoveAttribute(entityDecl, new TopLevelTypeName("System.Runtime.CompilerServices", "IteratorStateMachineAttribute"));
 			}
 		}

ICSharpCode.Decompiler/Tests/TestCases/Correctness/YieldReturn.cs

@@ -309,6 +309,11 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Correctness
 			}
 		}
 
+		public static IEnumerable<int> YieldBreakOnly()
+		{
+			yield break;
+		}
+		
 		public static IEnumerable<int> UnconditionalThrowInTryFinally()
 		{
 			// Here, MoveNext() doesn't call the finally methods at all