[mlir][SCF][NFC] `ValueBoundsConstraintSet`: Simplify `scf.for` implementation

This commit simplifies the implementation of the `ValueBoundsOpInterface` for `scf.for` based on the newly added `ValueBoundsConstraintSet::compare` API and adds additional documentation.

Previously, the interface implementation created a new constraint set just to check if the yielded value and iter_arg are equal. This was inefficient because constraints were added multiple times (to two different constraint sets) for ops that are inside the loop.