When creating a new JSRegExp for a literal, we sometimes create a
boilerplate and store it in the feedback vector. Then for future
creations, we can copy the boilerplate instead of re-creating the
regexp from scratch.

When we don't have a feedback vector, we currently create a
boilerplate, copy it and return the copy, and then throw out the
boilerplate, which is unnecessary. We can just return the first
JSRegExp we create.

Change-Id: I98b4e3a3082654ea989e0e6ba1524ce080b0125c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1776086Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63496}