glsl2: Clone methods return the type of the thing being cloned

This is as opposed to returning the type of the base class of the hierarchy.

src/glsl/ir_constant_expression.cpp

@@ -626,7 +626,7 @@ ir_constant_visitor::visit(ir_dereference_variable *ir)
 
    ir_variable *var = ir->variable_referenced();
    if (var && var->constant_value)
-      value = (ir_constant *)var->constant_value->clone(NULL);
+      value = var->constant_value->clone(NULL);
 }