Refactoring: improve performance for isVariableLive method, instead of using FluentIterables and filters everytime, we create a String representation of the global variables and the live variables object.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@14688 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c