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>

src/glsl/ir.h

@@ -789,6 +789,8 @@ enum ir_expression_operation {
    ir_unop_i2b,      /**< int-to-boolean conversion */
    ir_unop_b2i,      /**< Boolean-to-int conversion */
    ir_unop_u2f,      /**< Unsigned-to-float conversion. */
+   ir_unop_i2u,      /**< Integer-to-unsigned conversion. */
+   ir_unop_u2i,      /**< Unsigned-to-integer conversion. */
    ir_unop_any,
 
    /**