X64: Refactor the string add in the type recording binary operation stub...

src/x64/code-stubs-x64.h

Index: src/x64/code-stubs-x64.h
===================================================================
--- src/x64/code-stubs-x64.h	(revision 6881)
+++ src/x64/code-stubs-x64.h	(working copy)
@@ -360,24 +360,35 @@
 // Flag that indicates how to generate code for the stub StringAddStub.
 enum StringAddFlags {
   NO_STRING_ADD_FLAGS = 0,
-  NO_STRING_CHECK_IN_STUB = 1 << 0  // Omit string check in stub.
+  // Omit left string check in stub (left is definitely a string).
+  NO_STRING_CHECK_LEFT_IN_STUB = 1 << 0,
+  // Omit right string check in stub (right is definitely a string).
+  NO_STRING_CHECK_RIGHT_IN_STUB = 1 << 1,
+  // Omit both string checks in stub.
+  NO_STRING_CHECK_IN_STUB =
+      NO_STRING_CHECK_LEFT_IN_STUB | NO_STRING_CHECK_RIGHT_IN_STUB
 };
 
 
 class StringAddStub: public CodeStub {
  public:
-  explicit StringAddStub(StringAddFlags flags) {
-    string_check_ = ((flags & NO_STRING_CHECK_IN_STUB) == 0);
-  }
+  explicit StringAddStub(StringAddFlags flags) : flags_(flags) {}
 
  private:
   Major MajorKey() { return StringAdd; }
-  int MinorKey() { return string_check_ ? 0 : 1; }
+  int MinorKey() { return flags_; }
 
   void Generate(MacroAssembler* masm);
 
-  // Should the stub check whether arguments are strings?
-  bool string_check_;
+  void GenerateConvertArgument(MacroAssembler* masm,
+                               int stack_offset,
+                               Register arg,
+                               Register scratch1,
+                               Register scratch2,
+                               Register scratch3,
+                               Label* slow);
+
+  const StringAddFlags flags_;
 };