Can a license to a patent be considered a company's IP or asset? What can constitute as a company's IP?
You need to distinguish ownership of IP and IP rights.
If you own a patent then you also own all of the rights unless you have given those rights to someone else in a license. E.g., you can grant someone else an exclusive right to practice the patent and then you can't do it yourself.
If you don't own a patent, then you can get a license for an exclusive or non-exclusive right to practice the patent. In this situation you have IP rights but not ownership of the IP itself.
Both a patent and a license to a patent are assets.