X87: Simplify string copy in SubStringStub.

src/x87/code-stubs-x87.h

Index: src/x87/code-stubs-x87.h
===================================================================
--- src/x87/code-stubs-x87.h	(revision 21759)
+++ src/x87/code-stubs-x87.h	(working copy)
@@ -38,12 +38,12 @@
   // Generate code for copying characters using the rep movs instruction.
   // Copies ecx characters from esi to edi. Copying of overlapping regions is
   // not supported.
-  static void GenerateCopyCharactersREP(MacroAssembler* masm,
-                                        Register dest,     // Must be edi.
-                                        Register src,      // Must be esi.
-                                        Register count,    // Must be ecx.
-                                        Register scratch,  // Neither of above.
-                                        bool ascii);
+  static void GenerateCopyCharacters(MacroAssembler* masm,
+                                     Register dest,
+                                     Register src,
+                                     Register count,
+                                     Register scratch,
+                                     String::Encoding encoding);
 
   // Generate string hash.
   static void GenerateHashInit(MacroAssembler* masm,