UnreachableCodeChecker cleanup and improvements
- Fixed some iterator style issues
- Don't process blocks that have been visited already
- Fixed a case where a unreachable block cycle was not reported
- Minor test case changes
- Added one test case from flow-sensitive version of the check. More coming.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@115861 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed