Missing `invalid multibyte escape` SyntaxError for Regexp

Combination of Regexp encoding options and forced encodings (I suppose) should lead to the SyntaxError:

```ruby
# encoding: us-ascii
p /\xFF/u.encoding
```

```
ruby script.rb
script.rb: --> script.rb
invalid multibyte escape: /\xFF/
> 3  p /\xFF/u.encoding
script.rb:3: invalid multibyte escape: /\xFF/ (SyntaxError)
```