AllocateScopeInfos should only be called for not-yet unallocated scopes,
so we can DCHECK for nullness rather than having a branch for it.

Change-Id: I65767c22d6fd7b12c4564c5ecc52f9486229affa
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1981159Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65623}