It would be nice if unused {RegClass}es just equal {kNoReg} on all
platforms.
This CL adds the static asserts for that, and slightly modifies the
definition of the {RegClass} enum to fulfil that. Interestingly, this
only changes the values for the one configuration that is currently
unused :)

R=zhin@chromium.org

Change-Id: Ib1b2a663183ad34822f514d6168c65450c1d7f2e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1975753Reviewed-by: Zhi An Ng <zhin@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65527}