[CodeGen] MachineFunctionSplitter: Add missing initializer (#123564)

This registers the pass with PassRegistry so we can use -start-before
and other options for machine-function-splitter.

Author: optimisan

llvm/lib/CodeGen/CodeGen.cpp

@@ -82,6 +82,7 @@ void llvm::initializeCodeGen(PassRegistry &Registry) {
   initializeMachineCycleInfoWrapperPassPass(Registry);
   initializeMachineDominatorTreeWrapperPassPass(Registry);
   initializeMachineFunctionPrinterPassPass(Registry);
+  initializeMachineFunctionSplitterPass(Registry);
   initializeMachineLateInstrsCleanupPass(Registry);
   initializeMachineLICMPass(Registry);
   initializeMachineLoopInfoWrapperPassPass(Registry);