Whenever the deal closes, it's my understanding that CA people will be given an offer to be retained or they'll be informed they're being laid off. If you're being laid off, maybe it's right then, maybe after whatever warning period is required at your location. If you get an offer and you reject it, It's my understanding that you won't get severance, because you're quitting.
I bet anyone who thinks they have an "option" to be retained or to take severance, that'll be managers moving people up and down their recommendation lists. It's CA people who will recommend who stays and who goes, not Broadcom people, because they don't know who knows anything. Broadcom may have a target headcount to meet in each area, but I don't believe they're going to pick at random.