We currently have a BitVector implementation which is used a lot by the
two (mid-tier and top-tier) register allocators. Their size is the
number of virtual registers or the number of blocks in the function. If
one of those numbers gets huge, the BitVector does not perform well any
more, and it consumes huge amounts of memory (we see up to several GBs
for huge Wasm functions).

This CL introduces a SparseBitVector implementation with a compatible
interface, meant to replace the BitVector implementation. Usages will be
introduced in follow-up CLs, first for the mid-tier allocator, then
top-tier. This will allow us to assess performance changes better, and
revert individual usages.

R=mslekova@chromium.org

Bug: chromium:1313379, v8:12780
Change-Id: I804311e0c188526961f70e88a43dd1ea26497cda
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3634780
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80546}