This uses a SparseBitVector instead of a BitVector for storing sets of
blocks. As we only use the mid-tier register allocator for huge
functions, this should generally be a win in both compile time and
memory usage.

R=mslekova@chromium.org

Bug: chromium:1313379, v8:12780
Change-Id: Icf5b50c62f1c5fd69877cd54833d9dea8d1c37e1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3634781Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80554}