This ports RegExp.prototype.exec to a TurboFan builtin.

LastMatchInfo is now stored on the context in order to be able to access
it from the stub.

Unmodified RegExp instances go through a fast path of accessing the
lastIndex property as an in-object field, while modified instances call
into runtime for lastIndex loads and stores.

Octane/regexp shows slight improvements (between 0 and 5%) with this CL.

BUG=v8:5339

Review-Url: https://codereview.chromium.org/2375953002
Cr-Commit-Position: refs/heads/master@{#39899}