Revert of [regexp] Remove dead code (patchset #1 id:1 of https://codereview.chromium.org/2301803003/ )

Reason for revert:
Performance regressions: crbug.com/644087
Clusterfuzz: crbug.com/644074

We'll reland all regexp changes at once when the port is complete and at least performance-neutral, since the partial port requires slow workarounds.

Original issue's description:
> [regexp] Remove dead code
>
> Remove dead code, and drop the legacy RegExp.prototype.exec implementation (the
> only differences are TO_BOOLEAN on global and sticky).
>
> BUG=v8:5339
>
> Committed: https://crrev.com/1cb2a70cb5baa7545fdf9cc4e035a6d45cb189fd
> Cr-Commit-Position: refs/heads/master@{#39121}

TBR=bmeurer@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=v8:5339

Review-Url: https://codereview.chromium.org/2310903002
Cr-Commit-Position: refs/heads/master@{#39165}