Ensure CFG invariants hold and expand invariants

[wmdietl]

The CFG invariants are encoded here:

https://github.com/eisop/checker-framework/blob/811e15dfd208ef5f0e5b1d6ed19e62c604045a0b/dataflow/src/main/java/org/checkerframework/dataflow/cfg/ControlFlowGraph.java#L151

At the moment they only run on one test:

https://github.com/eisop/checker-framework/blob/master/dataflow/src/test/java/cfgconstruction/CFGConstruction.java

Add a command-line option to check the invariants on a normal run and add a CI build that checks the invariants in all tests.

Look through the invariants and expand them if necessary.

[wmdietl]

In particular, for the test case from #692, the empty block for the loop body should have two predecessors: the condition and the loop body. It appears that the condition has the loop body as successor, but the loop body does not have the condition as predecessor.