fixed loop-related bugs: (1) getting loop head for a BasicBlock, (2) getting blocks for a loop body, (3) dropping path constraints in a loop
replaced "stale constraints" refutations with more direct and immediate refutations based on null parameter binding
removing Z3 note
working on making refutations based on stale constraints more transparent. in most (all?) cases these refutations are legitimate, but they could happen earlier, which would make the reason for refutations much more clear. have fixed this issue for all tests and also fixed a bug for calculating loop body for do...while loops. also added test for this issue.