Fix build break of VS 2013 debug builds

In debug mode extra macros are enabled for several C++ algorithms. Some of them
may cause unfortunate build failures.

This commit adds a redundant operator() to work around one of those troublesome
macros which was hit accidentally by change r278012.

llvm-svn: 278241

lib/Target/SystemZ/AsmParser/SystemZAsmParser.cpp

@@ -517,6 +517,9 @@ struct CompareInsn {
   bool operator() (StringRef LHS, const InsnMatchEntry &RHS) {
     return LHS < RHS.Format;
   }
+  bool operator() (const InsnMatchEntry &LHS, const InsnMatchEntry &RHS) {
+    return LHS.Format < RHS.Format;
+  }
 };
 
 // Table initializing information for parsing the .insn directive.