Use generic CFG splitting to add a new type of split for exception handlers,
`ExceptionHandlerSplit`, which tags eachs node belonging to a `catch` clause
with the type of exception being caught. This allows for a more accurate CFG
for `try-catch` statements, where exception filters are handled properly.
Refactor existing logic for splitting control flow nodes belonging to a `finally`
block. A `Split` defines (1) when to enter the split, (2) when to stay in the split,
and (3) when to leave the split. With only these definitions, control flow splitting
is achieved by tagging each control flow element with the set of splits that apply
to it.
