Ping An Assurance (Group) Co. said it is branching out to invest in a bank Shenzhen and in expressway construction in northern China, as first major cross-sector domestic investments by a Chinese insurance company, the Wall Street Journal reported.   Ping An, the country's second-largest insurer by premiums after China Life Insurance Co., said it will pay 4.9 billion yuan for an 89 percent stake in Shenzhen Commercial Bank. It also said it signed a letter of intent to buy stakes in three expressways in Shanxi province, said the paper.   While the government lifted restrictions on insurance companies buying stakes from banks earlier this year, banks are also allowed to offer insurance products. Insurers aare also allowed to invest in infrastructure projects.