This removes {CPURegister::Is} and {CPURegister::is}, and just uses
{CPURegister::operator==} instead.

Drive-by: Use DCHECK_EQ and DCHECK_NE where possible.

R=mstarzinger@chromium.org

Bug: v8:9810
Change-Id: I03aad8b4223bd4ae37d468326a734f7a5c3c8061
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1916202Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64956}