The number of registers is very small, so any integer type is big
enough anyway. Just use the simplest one, which is int.

R=ahaas@chromium.org

Bug: v8:6600
Change-Id: I9762a5c934848b810b7937ee603cd183f883d557
Reviewed-on: https://chromium-review.googlesource.com/c/1394548Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58524}