[compiler] Properly validate stable map assumption for globals.

src/crankshaft/hydrogen.cc

Index: src/crankshaft/hydrogen.cc
diff --git a/src/crankshaft/hydrogen.cc b/src/crankshaft/hydrogen.cc
index 16c3639a3d33dbaebaecfa71aeb90bbc11242ff2..79e78a53f5b32c2dce5ad4f9b95b4858e7d88100 100644
--- a/src/crankshaft/hydrogen.cc
+++ b/src/crankshaft/hydrogen.cc
@@ -6518,11 +6518,19 @@ void HOptimizedGraphBuilder::HandleGlobalVariableAssignment(
           access = access.WithRepresentation(Representation::Smi());
           break;
         case PropertyCellConstantType::kStableMap: {
-          // The map may no longer be stable, deopt if it's ever different from
-          // what is currently there, which will allow for restablization.
-          Handle<Map> map(HeapObject::cast(cell->value())->map());
+          // First check that the previous value of the {cell} still has the
+          // map that we are about to check the new {value} for. If not, then
+          // the stable map assumption was invalidated and we cannot continue
+          // with the optimized code.
+          Handle<HeapObject> cell_value(HeapObject::cast(cell->value()));
+          Handle<Map> cell_value_map(cell_value->map());
+          if (!cell_value_map->is_stable()) {
+            return Bailout(kUnstableConstantTypeHeapObject);
+          }
+          top_info()->dependencies()->AssumeMapStable(cell_value_map);
+          // Now check that the new {value} is a HeapObject with the same map.
           Add<HCheckHeapObject>(value);
-          value = Add<HCheckMaps>(value, map);
+          value = Add<HCheckMaps>(value, cell_value_map);
           access = access.WithRepresentation(Representation::HeapObject());
           break;
         }