It's actually also in the icon picker dropdown -- second option down the list, just under "Default". When you pick it, the button does come up for you to roll again if you want a new pick.

(The changes we're making to the comment form are to improve the mobile experience -- we're trying to get the comment form down to one screen both horizontally and vertically, and that involves a lot of space-juggling. The random button isn't incredibly frequently used, but it is something the people who use really love, so we didn't want to lose it completely -- but we needed to steal its space!)