[in-place weak refs] Write the CSA::DispatchMaybeObject check differently.

The previous version was correct too, since we check sminess before. But with the
new check, it's easier to see it's correct.

BUG=v8:7308

Change-Id: I1632353ee5dfd305479858ec4a690b17bb70e6a6
Reviewed-on: https://chromium-review.googlesource.com/1039525Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53099}

src/code-stub-assembler.cc

@@ -1690,8 +1690,8 @@ void CodeStubAssembler::DispatchMaybeObject(TNode<MaybeObject> maybe_object,
          if_cleared);
 
   GotoIf(WordEqual(WordAnd(BitcastMaybeObjectToWord(maybe_object),
-                           IntPtrConstant(kWeakHeapObjectMask)),
-                   IntPtrConstant(0)),
+                           IntPtrConstant(kHeapObjectTagMask)),
+                   IntPtrConstant(kHeapObjectTag)),
          &inner_if_strong);
 
   *extracted =