ir_print_visitor: Print (constant bool (1)) instead of "true"

It might be better to simply handle "true" in the reader, but since
booleans normally aren't printed as "true" or "false", we may as well go
for consistency.

ir_print_visitor.cpp

@@ -218,7 +218,7 @@ void ir_print_visitor::visit(ir_assignment *ir)
    if (ir->condition)
       ir->condition->accept(this);
    else
-      printf("true");
+      printf("(constant bool (1))");
 
    printf(" ");