There are several HLSL syntax highlight add-ins for Visual Studio out there, but if you prefer to use the great NotePad++ to author or edit your shaders, my fellow DirectX MVP Matt Pettineo has written a Notepad++ add-in to allow doing this.
You just need to download the HLSL.xml file from his GoogleDrive account, and in Notepad ++ click Language->Define Your Language->Import and then select the downloaded file. After restarting notepad++, you´ll find a new entry in the Language menu item like this:
By clicking in that new item when you load an HLSL file, you´ll get the following result:
According to him, it supports even SM 5.0 profiles.
Great job Matt !!! :)