nir/lower_double_ops: lower round_even()

At least i965 hardware does not have native support for round_even() on doubles.

Reviewed-by: Matt Turner <mattst88@gmail.com>

src/compiler/nir/nir.h

@@ -2420,7 +2420,8 @@ typedef enum {
    nir_lower_dtrunc = (1 << 3),
    nir_lower_dfloor = (1 << 4),
    nir_lower_dceil = (1 << 5),
-   nir_lower_dfract = (1 << 6)
+   nir_lower_dfract = (1 << 6),
+   nir_lower_dround_even = (1 << 7)
 } nir_lower_doubles_options;
 
 void nir_lower_doubles(nir_shader *shader, nir_lower_doubles_options options);