glsl: Fix cut-and-paste bug in hierarchical visitor ir_expression::accept

At this point in the code, s must be visit_continue.  If the child
returned visit_stop, visit_stop is the only correct thing to return.

Found by inspection.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/compiler/glsl/ir_hv_accept.cpp

@@ -147,7 +147,7 @@ ir_expression::accept(ir_hierarchical_visitor *v)
 	 goto done;
 
       case visit_stop:
-	 return s;
+	 return visit_stop;
       }
    }