Port string keyed load IC improvements (r4444) to x64.

src/ia32/codegen-ia32.h

Index: src/ia32/codegen-ia32.h
===================================================================
--- src/ia32/codegen-ia32.h	(revision 4526)
+++ src/ia32/codegen-ia32.h	(working copy)
@@ -886,14 +886,15 @@
 class StringHelper : public AllStatic {
  public:
   // Generates fast code for getting a char code out of a string
-  // object at the given index. May bail out for three reasons (in the
+  // object at the given index. May bail out for four reasons (in the
   // listed order):
   //   * Receiver is not a string (receiver_not_string label).
-  //   * Index is not a positive smi (index_not_positive_smi label).
+  //   * Index is not a smi (index_not_smi label).
+  //   * Index is out of range (index_out_of_range).
   //   * Some other reason (slow_case label). In this case it's
   //     guaranteed that the above conditions are not violated,
   //     e.g. it's safe to assume the receiver is a string and the
-  //     index is a positive smi.
+  //     index is a non-negative smi < length.
   // When successful, object, index, and scratch are clobbered.
   // Otherwise, scratch and result are clobbered.
   static void GenerateFastCharCodeAt(MacroAssembler* masm,
@@ -902,7 +903,8 @@
                                      Register scratch,
                                      Register result,
                                      Label* receiver_not_string,
-                                     Label* index_not_positive_smi,
+                                     Label* index_not_smi,
+                                     Label* index_out_of_range,
                                      Label* slow_case);
 
   // Generates code for creating a one-char string from the given char