[regexp] Refactor RegExp.prototype.exec

This refactors portions of exec into a new function without RegExpResult
construction, which will be used in the future by test, @@match, and @@search
fast paths.

Unnecessary ToString and ToLength calls as well as repeated map checks were
removed.

BUG=v8:5339

Review-Url: https://codereview.chromium.org/2540153002
Cr-Commit-Position: refs/heads/master@{#41414}