Adjusted the semantics of assign checking in IdempotentOperationChecker

Adjusted the semantics of assign checking in IdempotentOperationChecker
- Fixed a regression where assigning '0' would be reported
- Changed the way self assignments are filtered to allow constant testing
- Added a test case for assign ops
- Fixed one test case where a function pointer was not considered constant
- Fixed test cases relating to 0 assignment

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@112501 91177308-0d34-0410-b5e6-96231b3b80d8