In particular: initial values of local ArchOpcode variables that
get overwritten anyways. Creating these variables uninitialized makes
that obvious.

Change-Id: Ia205b5397c60769a46bf28ed60b299ac652f4b28
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2470557
Auto-Submit: Georg Neis <neis@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Commit-Queue: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70526}