[wasm][arm][liftoff] Fix another GetUnusedRegister

Fixed: chromium:1340488
Change-Id: Id3da10dd13256dfc15a6fef4dc412b5d30ccc8cc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3735126


Reviewed-by: Manos Koukoutos <manoskouk@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Auto-Submit: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81455}