Since all other values are not used any more if we return, we don't
need to spill anything (as might happen during {PopToRegister}).
Instead, just load the top stack value into the return register(s).

R=titzer@chromium.org

Bug: v8:6600, v8:8423
Change-Id: Ibfd02d20191459c7b136ab9a48f0cf1a53b3385d
Reviewed-on: https://chromium-review.googlesource.com/c/1358391
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57995}