[IRCE] Enable decreasing loops of non-const bound

As a follow-up to r328480, this updates the logic for the decreasing
safety checks in a similar manner:
- CanBeMax is replaced by CannotBeMaxInLoop which queries
  isLoopEntryGuardedByCond on the maximum value.
- SumCanReachMin is replaced by isSafeDecreasingBound which includes
  some logic from parseLoopStructure and, again, has been updated to
  use isLoopEntryGuardedByCond on the given bounds.

Differential Revision: https://reviews.llvm.org/D44776

llvm-svn: 328613