[CodeGen] Add no-op machine function pass for test purpose (#79541)

Author: paperchalice

llvm/include/llvm/Passes/MachinePassRegistry.def

@@ -127,6 +127,7 @@ MACHINE_FUNCTION_ANALYSIS("pass-instrumentation", PassInstrumentationAnalysis,
 #endif
 // MACHINE_FUNCTION_PASS("free-machine-function", FreeMachineFunctionPass, ())
 // MACHINE_FUNCTION_PASS("mir-printer", PrintMIRPass, ())
+MACHINE_FUNCTION_PASS("no-op-machine-function", NoOpMachineFunctionPass, ())
 #undef MACHINE_FUNCTION_PASS
 
 // After a pass is converted to new pass manager, its entry should be moved from

llvm/include/llvm/Passes/PassBuilder.h

@@ -852,6 +852,14 @@ struct NoOpLoopPass : PassInfoMixin<NoOpLoopPass> {
   }
 };
 
+/// No-op machine function pass which does nothing.
+struct NoOpMachineFunctionPass
+    : public MachinePassInfoMixin<NoOpMachineFunctionPass> {
+  PreservedAnalyses run(MachineFunction &, MachineFunctionAnalysisManager &) {
+    return PreservedAnalyses::all();
+  }
+};
+
 /// No-op loop analysis.
 class NoOpLoopAnalysis : public AnalysisInfoMixin<NoOpLoopAnalysis> {
   friend AnalysisInfoMixin<NoOpLoopAnalysis>;