[mlir][intrange] Represent bounds of `ReflectBoundsOp` as `si`/`ui` (#92641)

This patch adapts the `test.reflect_bounds` test Op to use explicitly
signed and unsigned representation for signed and unsigned bounds of
`IntegerType`s.

This is mostly a cosmetic change as the internal representation of the
ranges is unchanged. However, it improves readability of tests.