Modernise the RegList interface to be a proper class, rather than a
typedef to an integer, and add proper methods onto it rather than ad-hoc
bit manipulation.

In particular, this makes RegList typesafe, adding a DoubleRegList for
DoubleRegisters.

The Arm64 CPURegList isn't updated to use (or extend) the new RegList
interface, because of its weird type-erasing semantics (it can store
Registers and VRegisters). Maybe in the future we'll want to get rid of
CPURegList entirely and use RegList/DoubleRegList directly.

Change-Id: I3cb2a4d386cb92a4dcd2edbdd3fba9ef71f354d6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3516747
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79460}