Extract common code in string compare loops.

src/x64/code-stubs-x64.h

Index: src/x64/code-stubs-x64.h
diff --git a/src/x64/code-stubs-x64.h b/src/x64/code-stubs-x64.h
index 0ee10b5f4b3dc172ee891ed61e7e764c6b3fa3e8..1842dc0a9608a5279a136da39310fbd3aa8c2ffc 100644
--- a/src/x64/code-stubs-x64.h
+++ b/src/x64/code-stubs-x64.h
@@ -384,10 +384,16 @@ class StringCompareStub: public CodeStub {
                                             Register scratch2);
 
  private:
-  Major MajorKey() { return StringCompare; }
-  int MinorKey() { return 0; }
+  virtual Major MajorKey() { return StringCompare; }
+  virtual int MinorKey() { return 0; }
+  virtual void Generate(MacroAssembler* masm);
 
-  void Generate(MacroAssembler* masm);
+  static void GenerateAsciiCharsCompareLoop(MacroAssembler* masm,
+                                            Register left,
+                                            Register right,
+                                            Register length,
+                                            Register scratch,
+                                            NearLabel* chars_not_equal);
 };