[llvm] [DXIL] Add support for root signature flag element in DXContainer (PR #123147)

[Justin Bogner via llvm-commits]

https://github.com/bogner commented:

We should add a couple of tests that use `opt` and print the analysis results rather than the `llc` and `obj2yaml`. These will be easier to write and uderstand. We should probably modify most of the error tests to do this as well to avoid running other unrelated passes before and after the root signature analysis.

There's a piece that I hadn't noticed you'd missed that's needed to do this though - we need to register the analysis in the new pass manager and also write a printer pass. You can take a look at DXILShaderFlags for the boilerplate for a printer pass, and it may be as simple as:

```c++
PreservedAnalyses RootSignatureAnalysisPrinter::run(Module &M,
                                                    ModuleAnalysisManager &AM) {
  std::optional<ModuleRootSignature> &MRS =
      AM.getResult<RootSignatureAnalysis>(M);
  if (MRS)
    OS << "Root Signature flags: " << format_hex(MRS->Flags, 8) << "\n";
  else
    OS << "Root Signature: not found";

  return PreservedAnalyses::all();
}
```

though we may want to print something a bit more human readable :)

For the registry, this is a matter of adding a `MODULE_ANALYSIS("dxil-root-signature", ...` and ` MODULE_PASS("print<dxil-root-signature>", ...` to DirectXPassRegistry.def

https://github.com/llvm/llvm-project/pull/123147