By Juby Babu Dec 11 (Reuters) - Broadcom projected first-quarter revenue above Wall Street estimates on Thursday, but said that margins would fall due to a higher mix of AI revenue, ​and shares fell 5% in extended trading. Broadcom has jumped into the AI chip business, which has ‌investors nervous about the profitability and costs of enormous investments. The company has a backlog of $73 billion that it anticipates shipping ‌over the next 18 months, CEO Hock Tan said on a post-earnings call, but his lieu ...