glsl: Add a constant_referenced method to ir_dereference*

glsl: Add a constant_referenced method to ir_dereference*

The method is used to get a reference to an ir_constant * within the
context of evaluating an assignment when calculating a
constant_expression_value.

Signed-off-by: Olivier Galibert <galibert@pobox.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net> [v1]