[TurboFan] Reduce zone memory usage in register-allocator-verifier.

Avoid allocating local objects in the outer zone, instead create a new inner zone
in ValidatePendingAssessment.

BUG=v8:5796

Review-Url: https://codereview.chromium.org/2617413002
Cr-Commit-Position: refs/heads/master@{#42151}

src/compiler/register-allocator-verifier.cc

@@ -373,8 +373,9 @@ void RegisterAllocatorVerifier::ValidatePendingAssessment(
   // for the original operand (the one where the assessment was created for
   // first) are also pending. To avoid recursion, we use a work list. To
   // deal with cycles, we keep a set of seen nodes.
-  ZoneQueue<std::pair<const PendingAssessment*, int>> worklist(zone());
-  ZoneSet<RpoNumber> seen(zone());
+  Zone local_zone(zone()->allocator(), ZONE_NAME);
+  ZoneQueue<std::pair<const PendingAssessment*, int>> worklist(&local_zone);
+  ZoneSet<RpoNumber> seen(&local_zone);
   worklist.push(std::make_pair(assessment, virtual_register));
   seen.insert(block_id);