Just months after betting up to $1.58 billion on the drug discovery and early development chops of IFM Therapeutics LLC, Novartis AG is again investing in the Boston-based company. This time, it has agreed to pay IFM shareholders up to $840 million for an exclusive option to acquire a subsidiary they launched just months ago to develop new drugs for the treatment of inflammatory and autoimmune diseases, IFM Due (pronounced du-way) Inc.