glsl2: Define new ir_discard instruction.

src/glsl/ir_print_visitor.cpp

@@ -314,6 +314,20 @@ ir_print_visitor::visit(ir_return *ir)
 }
 
 
+void
+ir_print_visitor::visit(ir_discard *ir)
+{
+   printf("(discard ");
+
+   if (ir->condition != NULL) {
+      printf(" ");
+      ir->condition->accept(this);
+   }
+
+   printf(")");
+}
+
+
 void
 ir_print_visitor::visit(ir_if *ir)
 {