For Chevy, the Sonic is almost definitely going to go. The Impala has a high chance as well. Here are the reasons below. It isn't rocket science if you keep track of these cars and how it ties into the current market:
Sonic: (Totally dead)
I can see the Chevy Sonic being the first to go. It isn't really a longstanding legacy nameplate. It's just an American market version of the Aveo brought over in 2011, and hasn't seen anything more than a minor facelift in that time. Compact cars have generally had lukewarm response in the U.S. at best, and the surge of crossover sales in the last 5 years will be enough to kill it here.
Impala: (Probably dead)
Second in line to get the ax will be the Impala. While the 10th generation launched in MY2014 to critical acclaim over a much improved design, it was at a bad time for the sedan market, and sales are slumping as well due to crossovers. It's about due for a mid cycle refresh at this point, but it may not be worth it.
The MY2016 Chevy Malibu got a similar huge update that makes it a much nicer car and might have cannibalized some sales from the Impala. Also, the Malibu got a brand new hybrid system derived from the Volt which gives it better market sustainability. By comparison, the Impala had an "Eco" variant that was discontinued just one year into its life by MY2015 over mixed reviews stemming from a very poor implementation of its older hybrid system.
GM has readily killed the Impala in the past as the market dictates, and I think they will do it again. As before, it can come back though.
And finally
Why GM is hurting so badly right now:
Simply put, customers want crossovers and GM does not have any strong offerings. It only has the Trax and Equinox, neither of which are particularly well received. The Trax has been panned for its very plain design, poor build quality, and anemic performance (even for its respective segment.) It's often excluded from top ten lists, and when it does appear in them, comes almost dead last compared to strong mainstays like the Kia Soul. The Equinox is just very slightly larger than the Trax, and although it generally receives slightly better reviews, the larger size might take it out of consideration for those seeking smaller crossovers like the Soul.
GM is not playing the market very intelligently. One need only look at its competitor, Ford, for how to best take advantage of the current climate. Rather than kill off their compact, the Ford Fiesta, they planned ahead well for the newly-launched generation by flexibly adjusting their chassis designs to offer the car in multiple variants, including one that has many attributes of a crossover. It has an increased ride height and has been stretched out from a 2-door to a 4-door.
Here is the Ford Fiesta "Active." Ford is playing it smart. Stuff like this will keep car sales up across their lineup, and they are finally pushing hard to dramatically improve interiors across all of their vehicles as GM had begun doing a few model years ago.
Your move, GM.