[mlir][SCF] `ValueBoundsConstraintSet`: Support preliminary support for branches

This commit adds support for `scf.if` to `ValueBoundsConstraintSet`.

Example:
```
%0 = scf.if ... -> index {
  scf.yield %a : index
} else {
  scf.yield %b : index
}
```

The following constraints hold for %0:
* %0 >= min(%a, %b)
* %0 <= max(%a, %b)

Such constraints cannot be added to the constraint set; min/max is not supported by `IntegerRelation`. However, if we know which one of %a and %b is larger, we can add constraints for %0. E.g., if %a <= %b:
* %0 >= %a
* %0 <= %b

This commit required a few minor changes to the `ValueBoundsConstraintSet` infrastructure, so that values can be compared while we are still in the process of traversing the IR/adding constraints.