[Assignment Tracking] Fix faulty assertion inside std::sort predicate

The vectors being sorted here shouldn't contain duplicate entries. Prior to
this patch this was checked with an assert within the `std::sort`
predicate. However, `std::sort` may compare an element against itself which
causes the assert to fire (false positive). Move the assert outside of the sort
predicate to avoid such issues.

Reviewed By: StephenTozer

Differential Revision: https://reviews.llvm.org/D149045