Since SSAPhiInstructions are never visited by NullPointerTransferFunctionProvider.TransferFunctionSSAVisitor,
we now respect phi instructions present at a given block by providing additional NodeTransferFunctions, improving precision.
Formerly, meets would lead to incorrect results due to incorrect initialization of initial data flow facts.
These are now properly initialized, interpreting
"State.BOTH" to mean: both "null" and "non-null" are possible values for the given variable, and
"State.UNKNOWN" to be the absurd assertion.
The initial fact at the entry block assumes variables to be BOTH, other blocks are initialy assumed unreachable and hence their variables to be UNKNOWN.
Martin Hecker
Julian Dolby
Juergen Graf