ScopDetection: Add new SCEV Validator

The SCEV Validator is used to check if the bound of a loop can be translated
into a polyhedral constraint. The new validator is more general as the check
used previously and e.g. allows bounds like 'smax 1, %a'. At the moment, we
only allow signed comparisons. Also, the new validator is only used to verify
loop bounds. Memory accesses are still handled by the old validator.

llvm-svn: 143576