Instead of scanning the array of registers, keep an explicit list of
free registers. Stack slots are equally changed to use an std::vector of
free slots instead of a linked list. Now we only need to scan
- the list of free registers when we want to allocate a specific
  register,
- and scan the list of allocated registers to see if the free value is
  already in a different register,
- scan the list of allocated registers to free some register if we don't
  have enough registers (for input, output, or temp).

Bug: v8:7700
Change-Id: Iff41b06aae656b59e4ed25e9066671a21660a73e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3489487Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Auto-Submit: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79278}