[SCEV] Limited support for unsigned preds in isImpliedViaOperations

The logic there only considers `SLT/SGT` predicates. We can use the same logic
for proving `ULT/UGT` predicates if all involved values are non-negative.

Adding full-scale support for unsigned might be challenging because of code amount,
so we can consider this in the future.

Differential Revision: https://reviews.llvm.org/D88087
Reviewed By: reames