Our current Freelist uses 47 categories. Because FreeListCategory is
about 26 bytes or so, this causes a non-neglieable overhead per
page. This CL divides by two the number of categories used.

Benchmark have shown that this should not increase old heap size. And
since less categories will be used, size of mallocated objects will be
smaller.
Furthermore,in both precise and fast modes, Scavenger time is lower
when using 24 Freelists rather than 47.

Bug: v8:9329
Change-Id: I81cca63a0ffd0aad5f9e009a176f401f2299493d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1778941
Commit-Queue: Darius Mercadier <dmercadier@google.com>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63483}