Using array index hash code for string-to-number conversion.

src/x64/macro-assembler-x64.h

Index: src/x64/macro-assembler-x64.h
===================================================================
--- src/x64/macro-assembler-x64.h	(revision 5357)
+++ src/x64/macro-assembler-x64.h	(working copy)
@@ -723,6 +723,12 @@
   // occurred.
   void IllegalOperation(int num_arguments);
 
+  // Picks out an array index from the hash field.
+  // Register use:
+  //   hash - holds the index's hash. Clobbered.
+  //   index - holds the overwritten index on exit.
+  void IndexFromHash(Register hash, Register index);
+
   // Find the function context up the context chain.
   void LoadContext(Register dst, int context_chain_length);