Disable double const store check to see impact on canary

R=tebbi@chromium.org

Bug: chromium:964833
Change-Id: I798f7c38eacaa16011ab7cc9ac4dea066078fbb5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1643170Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Georg Schmid <gsps@google.com>
Cr-Commit-Position: refs/heads/master@{#61977}

src/compiler/load-elimination.cc

@@ -923,18 +923,20 @@ Reduction LoadElimination::ReduceStoreField(Node* node,
       FieldInfo const* lookup_result =
           state->LookupField(object, field_index, constness);
 
-      if (lookup_result) {
+      if (lookup_result && constness == PropertyConstness::kMutable) {
         // At runtime, we should never encounter
         // - any store replacing existing info with a different, incompatible
         //   representation, nor
         // - two consecutive const stores.
         // However, we may see such code statically, so we guard against
         // executing it by emitting Unreachable.
+        // TODO(gsps): Re-enable the double const store check once we have
+        //   identified other FieldAccesses that should be marked mutable
+        //   instead of const (cf. JSCreateLowering::AllocateFastLiteral).
         bool incompatible_representation =
             !lookup_result->name.is_null() &&
             !IsCompatible(representation, lookup_result->representation);
-        if (incompatible_representation ||
-            constness == PropertyConstness::kConst) {
+        if (incompatible_representation) {
           Node* control = NodeProperties::GetControlInput(node);
           Node* unreachable =
               graph()->NewNode(common()->Unreachable(), effect, control);