Make collapsing of "x = x + y" to "x += y" work in a few more cases.

ICSharpCode.Decompiler/Ast/Transforms/ReplaceMethodCallsWithOperators.cs

@@ -207,7 +207,14 @@ namespace ICSharpCode.Decompiler.Ast.Transforms
 		
 		bool IsWithoutSideEffects(Expression left)
 		{
-			return left is IdentifierExpression; // TODO
+			if (left is ThisReferenceExpression)
+				return true;
+			if (left is IdentifierExpression)
+				return true;
+			MemberReferenceExpression mre = left as MemberReferenceExpression;
+			if (mre != null)
+				return mre.Annotation<FieldReference>() != null && IsWithoutSideEffects(mre.Target);
+			return false;
 		}
 		
 		void IAstTransform.Run(AstNode node)