Codegen bug for enum autocast in a type declaration 

A type declaration with an enum autocast causes a codegen error:

```cr
enum Foo
  Bar
end

y : Foo = :bar # Module validation failed: Load operand must be a pointer.
               #  %30 = load %"(Foo | Symbol)", i32 0, align 8, !dbg !105
```

Related: #12063