This CL removes the default parameter of GetUnusedRegister to avoid bugs
where the default parameter is used accidentially. With "{}" the default
value of the parameter is easy to write, and also not much more difficult to read.

R=clemensb@chromium.org

Bug: v8:10506
Change-Id: I3debe5eb91578c82abdac81dc6c252435fdf30d6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2202991Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67822}