UserAgent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:59.0) Gecko/20100101 Firefox/59.0

Steps to reproduce the problem:
Run the following code in TurboFan:

```
const regex = / /g;

benchmark(); // ~750ms
regex.someProp = 1; // Makes this one RegExp slower
// RegExp.prototype.someProp = 1; // Makes ALL RegExp slower!
benchmark(); // ~20s

function benchmark() {
  const start = Date.now();
  const words = 'a a a a a a a a a a a a a a a a a a ' +
                'a a a a a a a a a a a a a a a a a';
  for (let i = 0; i < 1000000; i++) {
    void words.replace(regex, "|");
  }
  const duration = Date.now() - start;
  console.log(`operation took ${duration}ms`);
}
```

What is the expected behavior?
I would expect that the speed of the RegExp would be the same before and after adding a property.

What went wrong?
The operation became slower when adding an additional property to the RegExp.

Did this work before? N/A 

Chrome version: Version 67.0.3396.40 (Official Build) beta (64-bit)<Copy from: 'about:version'>  Channel: beta
OS Version: Version 67.0.3396.40 (Official Build) beta (64-bit)
Flash Version: Shockwave Flash 29.0 r0

Seems to be related to this code path:
https://github.com/nodejs/node/blob/v8.11.1/deps/v8/src/builtins/builtins-regexp-gen.cc#L912-L929

It makes sense that if a RegExp has changed in such a way that it would affect the pattern matching, such as swapping out the .exec method, that the optimized operation would have to be skipped.

However, when adding an unrelated property, it might still be safe to still take the optimized path.