[stubs] KeyedLoadIC_Megamorphic: use stub cache lookup as fallback

When the inline lookup on a fast-property receiver doesn't find anything,
try a stub cache lookup before going into the runtime. This is much
faster for properties loaded from the receiver's prototype chain.

BUG=chromium:650236

Review-Url: https://codereview.chromium.org/2373683002
Cr-Commit-Position: refs/heads/master@{#39813}

src/code-stub-assembler.cc

@@ -4470,7 +4470,7 @@ void CodeStubAssembler::KeyedLoadICGeneric(const LoadICParameters* p) {
     Variable var_name_index(this, MachineType::PointerRepresentation());
     Label if_descriptor_found(this);
     DescriptorLookupLinear(key, descriptors, nof, &if_descriptor_found,
-                           &var_name_index, &slow);
+                           &var_name_index, &stub_cache);
 
     Bind(&if_descriptor_found);
     {

src/ic/ic.cc

@@ -1436,6 +1436,7 @@ MaybeHandle<Object> KeyedLoadIC::Load(Handle<Object> object,
   if (!is_vector_set()) {
     ConfigureVectorState(MEGAMORPHIC, key);
     TRACE_GENERIC_IC(isolate(), "KeyedLoadIC", "set generic");
+    TRACE_IC("LoadIC", key);
   }
 
   if (!load_handle.is_null()) return load_handle;