This requires changing the CFG trees for classes and modules from
post-order to pre-order so that we can place the writes at the root node
of the tree, to prevent them overlapping with reads in the body of the
class/module.
We need to do this because classes and modules don't define their own
basic block, but re-use the surrounding one. This problem doesn't occur
for `self` variables in methods because each method has its own basic
block and we can place the write on the entry node of the bock.
