Fix build with libcxx22
https://github.com/llvm/llvm-project/pull/160804

Index: llvm/include/llvm/CodeGen/RDFGraph.h
--- llvm/include/llvm/CodeGen/RDFGraph.h.orig
+++ llvm/include/llvm/CodeGen/RDFGraph.h
@@ -447,7 +447,7 @@ struct NodeAllocator { (private)
   AllocatorTy MemPool;
 };
 
-using RegisterSet = std::set<RegisterRef>;
+using RegisterSet = std::set<RegisterRef, RegisterRefLess>;
 
 struct TargetOperandInfo {
   TargetOperandInfo(const TargetInstrInfo &tii) : TII(tii) {}
