changes towards a canonical representation of comparisons: * new createNullCheckGuard helper method on CiLoweringTool * replaced NullCheckNode with IsNullNode * GuardNode and FixedGuardNode can be negated * keep a list of conditions that are true/false in CheckCastEliminationPhase * FixedGuardNode has only one condition * GraphBuilderPhase creates canonical CompareNodes * BooleanNodes can negate their usages * added junit test for canonicalized compares * removed junit test for negated instanceof * added more thorough graph comparison for junit tests * CheckCastEliminationPhase keeps track of conditions that are known to be true/false