These map checks were implemented for TF code already. This CL makes
sure that parts implemented in C++ follow the same logic, which is:

An object is an unmodified regexp if:
1) it's a receiver,
2) its map is the initial regexp map,
3) its prototype is a receiver,
4) and its prototype's map is the initial prototype's initial map.

We can now be smarter in @@replace and @@split since checking maps
(unlike the previous check of RegExp.prototype.exec) is not observable,
so we can perform fast-path checks at a time of our choosing.

BUG=v8:5339,v8:5434,v8:5123

Review-Url: https://chromiumcodereview.appspot.com/2434983002
Cr-Commit-Position: refs/heads/master@{#40495}