The bodylines within a procedure's bodylists are no longer marked as a TREE,
but there was still some code that tried to use and access the tree and this
no longer worked. This includes the logic for re-treeifying a procedure if
definitions changed while they're running.
This change also adds some asserts to find when a non-tree is treated as a TREE
and when a non-line is treated as a LINE. This should make it easier to find
and debug similar problems.