No reliable way to retrieve the IL of a function without using try/catch

[xusheng6]

While working on the binja backend for capa, I noticed a few outstanding crashes: mandiant/capa#2406 (comment), and mandiant/capa#2249. After troubleshooting this, I realized that to reliably get the IL of a function, we have to use try/catch for now.

Here is what could happen when I try to retrieve the IL of a function:

1. It is immediately available
2. It is not yet available, but can be retrieved briefly
3. It cannot be retrieved

Now think of the case of func.mlil, it will work for case 1 & 2, but it will throw an exception in case 3. If I use func.mlil_if_available, then it will work for case 1 & 3, but not for case 2 according to my test. I had to use this code pattern to handle all cases:

        try:
            mlil = f.mlil
        except ILException:
            return

        if mlil is None:
            return

        for mlil_bb in mlil.basic_blocks:
            pass

Which is quite ugly and inconvenient. It would be great if we can devise a way to retrieve the IL without needing to write code in the above pattern. Potentially we could just create a new API that wraps the above code logic

[xusheng6]

Related to #6020