glsl: Add ir_unop_i2u and ir_unop_u2i operations.

glsl: Add ir_unop_i2u and ir_unop_u2i operations.

These are necessary to handle int/uint constructor conversions.  For
example, the following code currently results in a type mismatch:

int x = 7;
uint y = uint(x);

In particular, uint(x) still has type int.

This commit simply adds the new operations; it does not generate them,
nor does it add backend support for them.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>