glsl: add a empty set_is_lhs on ast_node

Just to allow to call set_is_lhs on any ast_node without a casting. Useful
when processing a ast_node list that we know it contain ast_expression.

v2: comment out new_value to avoid unused parameter warning (Ian Romanick)

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/compiler/glsl/ast.h

@@ -126,6 +126,8 @@ public:
 
    exec_node link;
 
+   virtual void set_is_lhs(bool);
+
 protected:
    /**
     * The only constructor is protected so that only derived class objects can