Improved false positive detection and numerous small issues in UnreachableCodeChecker

- Reporting now uses getUnreachableStmt which returns the Stmt* we should report
- Indexing of reachable and visited blocks now use CFGBlock ID's instead of pointers
- The CFG used in the unreachable search is now the unoptimized CFG
- Added 'Dead code' category to warnings
- Removed obsolete function getCondition
- Simplified false positive detection based on properties of FindUnreachableEntryPoints

llvm-svn: 110148