glsl: Make ir_rvalue_visitor visit ir_discard::condition.

glsl: Make ir_rvalue_visitor visit ir_discard::condition.

This was forgotten.

I omitted the NULL check since we don't check ir_assignment::condition
either.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>