This ensures that we have enough slack to land or merge important fixes
(and temporarily coming closer to the limit).
If the static assertion is ever violated, we should immediately create a
tracking bug to free some opcode space. Temporarily reducing the
required slack (16 in this CL) is OK then.

R=zhin@chromium.org

Bug: v8:12093
Change-Id: I0934061c38cefb713ae83ccc4d81791dc4b2d312
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3097281Reviewed-by: Zhi An Ng <zhin@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76323}