Stack slots store the type of the respective value, so we can always
get the reg class from that type.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I60ca21daa4cc1505562c344367c7586bcd272c9b
Reviewed-on: https://chromium-review.googlesource.com/950948Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51820}