BoyerMoorePositionInfo is a simple wrapper around a bitset and an
associated ContainedInLattice field. This CL refactors bitset-related
operations that used to be implemented naively (e.g.: loop over all
bits to find a single set bit, or to generate a union of two bitsets).

Instead, use more suitable methods from base::bits and std::bitset.

Drive-by: Remove dead class members.
Drive-by: Zero the ByteArray with memset.

Bug: v8:9359
Change-Id: I33923c7d216320f4e3d7e4a6df2967f4aa86ab05
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1667407
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62419}