[scopes] Change null ScopeInfo if to DCHECK

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}

src/ast/scopes.cc

@@ -2481,7 +2481,9 @@ void DeclarationScope::RecordNeedsPrivateNameContextChainRecalc() {
 // static
 void DeclarationScope::AllocateScopeInfos(ParseInfo* info, Isolate* isolate) {
   DeclarationScope* scope = info->literal()->scope();
-  if (!scope->scope_info_.is_null()) return;  // Allocated by outer function.
+
+  // No one else should have allocated a scope info for this scope yet.
+  DCHECK(scope->scope_info_.is_null());
 
   MaybeHandle<ScopeInfo> outer_scope;
   if (scope->outer_scope_ != nullptr) {