Use the right register type for the temporary register.

R=titzer@chromium.org

Bug: v8:6600, chromium:917588
Change-Id: Ia2617f6b406924ca7f496608fd495faf04dff25b
Reviewed-on: https://chromium-review.googlesource.com/c/1403127Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58711}