I believe it is some kind of auto exposure adjust tool which you can use to adjust the Iray preview render on the fly. Click onto it and you will notice your cursor will change and you can draw a square over part of your scene to adjust the Exposure or just click and release to set exposure by a single pixel. You can also change the tool to adjust different settings in the Draw Style tab under advanced. The options include Exposure, White Balance or both. It has always been there as far as I am aware, but you probably have just never noticed it till you accidentally changed the settings in the view port.