[mlir][intrange] Represent bounds of `ReflectBoundsOp` as `si`/`ui`

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.

Example:
```mlir
// old:
test.reflect_bounds {smax = 127 : i8, smin = -128 : i8, umax = -56 : i8, umin = 100 : i8}
// new:
test.reflect_bounds {smax = 127 : si8, smin = -128 : si8, umax = 200 : ui8, umin = 100 : ui8}
```