Instead of ignoring boilerplate when decompiling `async for`, build the loop semantically by deconstructing the BLK_WHILE -> BLK_CONTAINER -> BLK_TRY that python generates when compiling an `async for` (see https://www.python.org/dev/peps/pep-0492/#asynchronous-iterators-and-async-for).
When reading GET_AITER, convert the BLK_WHILE into a BLK_ASYNCFOR in the same way as a `for` loop.
Ignore GET_ANEXT.
Use END_FINALLY as the trigger to finish building the BLK_ASYNCFOR:
-> Extract the loop variable from the contents of the BLK_TRY.
-> Throw away the BLK_TRY and BLK_CONTAINER, leave the BLK_ASYNCFOR on top of the stack.