[Docs] Update MergeBB usage to New Pass Manager syntax

[E00N777]

@banach-space

Summary

The README currently lists the Legacy Pass Manager command (-load, -legacy-merge-bb) for running the MergeBB pass. However, the codebase uses the New Pass Manager registration.

//-----------------------------------------------------------------------------
// New PM Registration
//-----------------------------------------------------------------------------
llvm::PassPluginLibraryInfo getMergeBBPluginInfo() {
  return {LLVM_PLUGIN_API_VERSION, "MergeBB", LLVM_VERSION_STRING,
          [](PassBuilder &PB) {
            PB.registerPipelineParsingCallback(
                [](StringRef Name, FunctionPassManager &FPM,
                   ArrayRef<PassBuilder::PipelineElement>) {
                  if (Name == "merge-bb") {
                    FPM.addPass(MergeBB());
                    return true;
                  }
                  return false;
                });
          }};
}

extern "C" LLVM_ATTRIBUTE_WEAK ::llvm::PassPluginLibraryInfo
llvmGetPassPluginInfo() {
  return getMergeBBPluginInfo();
}

This PR updates the command to use the New PM syntax:

• Changed -load to -load-pass-plugin
• Changed -legacy-merge-bb to -passes="merge-bb"

Validation

Verified locally that the new command works with LLVM-21 in ubuntu 22.04

Others

I also noticed something: the readme doesn't seem to mention where foo.ll comes from.
I Verified by duplicate.ll which generated by libduplicateBB.so. in the previous section(duplicateBB) like:

$ opt-21 -load-pass-plugin /home/ubuntu/llvm-tutor-debug/build/lib/libMergeBB.so -passes="merge-bb"  -S ./duplicate_bb.ll  -o duplicate_bb_merge.ll

It worked.
I didn't fixed it in this PR.Perhaps you have some suggestions?

[banach-space]

Thank you!

I didn't fixed it in this PR.Perhaps you have some suggestions?

Feel free to send a PR which updates README with a step to generate an input file.

Thanks for pointing it out!