Revert "glsl: replace unreachable code path with assert"

This reverts commit 98270fd20d.

Something went terribly wrong the commit is not what the commit
message says.

src/glsl/link_varyings.cpp

@@ -1295,12 +1295,13 @@ public:
 
    void process(ir_variable *var)
    {
-      /* All named varying interface blocks should be flattened by now */
-      assert(!var->is_interface_instance());
-
       this->toplevel_var = var;
       this->varying_floats = 0;
-      program_resource_visitor::process(var);
+      if (var->is_interface_instance())
+         program_resource_visitor::process(var->get_interface_type(),
+                                           var->get_interface_type()->name);
+      else
+         program_resource_visitor::process(var);
    }
 
 private: