This reverts commit 7a2651cb.

Reason for revert: https://ci.chromium.org/p/v8/builders/ci/V8%20Android%20Arm64%20-%20N5X/4126

Original change's description:
> [arm64] Cleanup TODO around handling of x18
> 
> Use `padreg` instead of x18 to maintain alignment in the CPURegList.
> 
> Also clean up some comments and tidy up RequiredStackSizeForCallerSaved
> and PushCallerSaved.
> 
> Change-Id: I80a780e5649e69a1746c43f37c2d1d875120c7a0
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1581609
> Reviewed-by: Jakob Gruber <jgruber@chromium.org>
> Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
> Cr-Commit-Position: refs/heads/master@{#60987}

TBR=jgruber@chromium.org,martyn.capewell@arm.com,joey.gouly@arm.com

Change-Id: Id95ac26142717f6503d284d20ca03b9de33a9122
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1582403Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60999}