Just use the default {is_valid} method provided by RegisterBase.

Drive-by: Also rename {CPURegList::IsValid} to {CPURegList::is_valid}.

R=mstarzinger@chromium.org

Bug: v8:9810
Change-Id: Ia3bc3c477e6329d63ffd00bca59762d9a6cf2fcc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1916201
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64952}