Instead of storing a single register, store the entire RegList. This
simplifies a lot of things. We will use RegLists for free registers etc
too later.

Bug: v8:7700
Change-Id: I32146023c7b9bc9e553e3db98fe034e8cef7d09d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3487994Reviewed-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@{#79289}