Extended calculated statistics

[janniclas]

This PR adds statistics for the following instruction counts:

• size_t Branches = 0;
• size_t GetElementPtrs = 0;
• size_t Terminators = 0;
• size_t PhiNodes = 0;

Further some dead code was removed.

[janniclas]

@fabianbs96 I included the changes we discussed in regards of exposing the complete GeneralStatistics from the IRDB class, as well as overloading the << operator in the statistics class to ease printing and cleanup the code in phasar-llvm.cpp