Runtime_CreateRegExpLiteral pretends to handle the case where we
already have a boilerplate but the code just passes an empty handle
to JSRegExp::Copy. This CL replaces the code with a CHECK that the
boilerplate doesn't exist.

A few mostly cosmetic changes along the way.

Change-Id: I856c77c1cdf58f09e920b773b66b037c5178edc7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1809357Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63852}