[stubs] KeyedLoadGeneric: support loading properties from strings

The spec for String.split() requires loading "separator[@@split]".
The KeyedLoadIC for this goes generic right away, but still requires
a runtime call. To avoid that, reshuffle checks in the generic stub
and do the primitive-string -> String-function translation inline.

Review-Url: https://codereview.chromium.org/2654893003
Cr-Commit-Position: refs/heads/master@{#42797}

src/ic/accessor-assembler.h

@@ -150,6 +150,15 @@ class AccessorAssembler : public CodeStubAssembler {
                                  Representation representation, Node* value,
                                  Node* transition, Label* miss);
 
+  // KeyedLoadIC_Generic implementation.
+
+  void GenericElementLoad(Node* receiver, Node* receiver_map,
+                          Node* instance_type, Node* index, Label* slow);
+
+  void GenericPropertyLoad(Node* receiver, Node* receiver_map,
+                           Node* instance_type, Node* key,
+                           const LoadICParameters* p, Label* slow);
+
   // Low-level helpers.
 
   Node* PrepareValueForStore(Node* handler_word, Node* holder,