Store the free registers as a RegList rather than stack of Register
values. This allows us to simplify some of the register freeing logic,
including passing the current free set to nodes for use as temporaries.

Drive-by: Replace ALWAYS_ALLOCATABLE_GENERAL_REGISTERS with
ALLOCATABLE_GENERAL_REGISTERS, which is the more general list (the former
is an implementation detail for optionally reserving a register for
the cage register).

Bug: v8:7700
Change-Id: I666e9a7547c2f4f4e578fbcbb4bd3fe3cb06dac5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3497767Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79344}