C#: Generic control flow graph splitting

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.

Author: hvitved

csharp/ql/src/semmle/code/csharp/controlflow/Completion.qll

@@ -123,11 +123,6 @@ class Completion extends TCompletion {
     this instanceof NormalCompletion or
     this instanceof ContinueCompletion
   }
-
-  /** Holds if this completion is a valid completion for exiting a callable. */
-  predicate isValidCallableExitCompletion() {
-    not this instanceof GotoCompletion
-  }
 }
 
 /** Holds if expression `e` has the Boolean constant value `value`. */