Although she was a regular fixture on NBC’s “The Tonight Show” when it was hosted by Johnny Carson throughout the 1980s, former fill-in host Joan Rivers hadn’t made an appearance on the program in 26 years. Rivers was never a guest during Jay Leno’s tenure as host, but made her first appearance as guest since Jimmy Fallon took over the show last month. Rivers did make a cameo in Fallon’s debut back in February…