refactoring SSA-index-calculation: now there is one single location (and not nine of them), where the real increment is done. The increment is a constant documentated value now. Also some other constants (VAR_UNSET, VAR_UNINITIALIZED) seem to depend on each other and on the increment.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@13184 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c