There's no reason to use the API RegExp type instead of the internal
JSRegExp type. In fact, the parsed flags end up in
Runtime_CreateRegExpLiteral, which assumes them to be of type
JSRegExp::Flags.

Drive-by: Additional asserts and helper functions in JSRegExp.

Bug: v8:9359
Change-Id: I5c12aba7d4e39a4891fb23d8b47c55fc480a28d9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1667004Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62376}