We generally try to use the most specific type, but sometimes we still
used LiftoffRegister even though it's statically known that it always
holds a GP register. Make these uses use Register directly. Note that
the conversion between Register and LiftoffRegister is a noop in
Release builds.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I1cf9aa727fb6fd71bbc1eed77df5e9d01e35ddee
Reviewed-on: https://chromium-review.googlesource.com/c/1278727Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56605}