[turbofan] Improve deferred code handling for polymorphic property access.

Only mark the last fallthrough control as deferred, otherwise the
splintering will ruin the code generation for the (maybe likely)
polymorphic cases.

Drive-by-fix: Reduce overall code duplication between JSLoadNamed
and JSStoreNamed specialization.

R=jarin@chromium.org
BUG=v8:4470
LOG=n

Review URL: https://codereview.chromium.org/1424733002

Cr-Commit-Position: refs/heads/master@{#31623}

src/compiler/js-native-context-specialization.h

@@ -68,6 +68,11 @@ class JSNativeContextSpecialization final : public AdvancedReducer {
                                   PropertyAccessMode access_mode,
                                   ZoneVector<PropertyAccessInfo>* access_infos);
 
+  Reduction ReduceNamedAccess(Node* node, Node* value,
+                              MapHandleList const& receiver_maps,
+                              Handle<Name> name,
+                              PropertyAccessMode access_mode);
+
   struct ScriptContextTableLookupResult;
   bool LookupInScriptContextTable(Handle<Name> name,
                                   ScriptContextTableLookupResult* result);