Enhance ConditionBRVisitor to write out more information

Hello!

I have found a problem in ConditionBRVisitor which is a 7 years old code made by Ted Kremenek: https://github.com/llvm-mirror/clang/commit/681bc114b51c1198cdec9a165c7d3230abb8f427

Sometimes it is impossible to write out what happens in a condition, only because of this code piece which is do nothing, but prevents information. After removing the GDM visiting some of the messages are duplicated because of the PostStmt visiting, where I have not found a way to remove the duplications. The patch where my project stucked: https://reviews.llvm.org/D53076

Thanks you for the suggestions,
Csaba.