glsl: Don't tree graft in the condition of an assignment

At this point, this should always be NULL.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14573>

src/compiler/glsl/opt_tree_grafting.cpp

@@ -177,8 +177,7 @@ ir_tree_grafting_visitor::check_graft(ir_instruction *ir, ir_variable *var)
 ir_visitor_status
 ir_tree_grafting_visitor::visit_leave(ir_assignment *ir)
 {
-   if (do_graft(&ir->rhs) ||
-       do_graft(&ir->condition))
+   if (do_graft(&ir->rhs))
       return visit_stop;
 
    /* If this assignment updates a variable used in the assignment