PTA: Change building pointer graph

[mchalupa]

• [ ] Get rid of paired nodes and stuff like that, it complicates the code
  • [x] add a new class for a part of graph instead of PSNodesSeq
• [ ] build CFG structure during building nodes (we now build them in dominance order, so we can do that)
• [ ] Use basic blocks to store nodes of the graph
• [ ] add fork/join edges (instead of regular CFG successors)
• [x] add call/return edges instead of regular CFG successors

[mchalupa]

WIP in pta-impr branch