This is a reland of 7a2651cb

x18 is not allocatable nor callee-saved in v8, so stop comparing
the before/after value in tests.

Presumably the Nexus failure was due to printf on that platform
clobbering x18.
This can be reproduced locally by modifying `CorruptAllCallerSavedCPURegister`
to also corrupt x18.

CQ_INCLUDE_TRYBOTS=luci.v8.try:v8_android_arm64_n5x_rel_ng

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}

Change-Id: I7c023a4706a98bcb9aa5acd37016a6d01e3979a6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1583762Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
Cr-Commit-Position: refs/heads/master@{#61078}