[MLIR][SCF] Removes incorrect assertion in loop unroller (#69028)

In particular, `upperBoundUnrolledCst` may be larger than `ubCst` when:

1. the step size is greater than 1;
2. `ub - lb` is not evenly divisible by the step size; and
3. the loop's trip count is evenly divisible by the unroll factor.

This is okay since the non-unit step size ensures that the unrolled loop
maintains the same trip count as the original loop. Added a test case
for this.

Fixes #61832.

Co-authored-by: Stephen Chou <stephenchou@google.com>