Avoid most of the {is_inline()} checks by having a {data_begin_} pointer
which either points at the inline storage or at the zone-allocated
memory.
This replaces a dynamic branch by a memory indirection, which is
beneficial for big (non-inline) BitVectors. For small BitVectors we will
have to see what the bots say; the hypothesis is that a memory load is
still faster than a dynamic branch.

Apart from better performance, this change allows for simpler code in
many places, including the iterator implementation.

R=jkummerow@chromium.org

Bug: v8:13063
Change-Id: I1e28279d1a438598e0b8403a6a4078c2cd2a4c48
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3776685Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81902}