Integrate LLVM at llvm/llvm-project@21d91a8ef319
Integrate LLVM at llvm/llvm-project@21d91a8ef319

Updates LLVM usage to match
[21d91a8ef319](https://github.com/llvm/llvm-project/commit/21d91a8ef319)

PiperOrigin-RevId: 391645196

google-research/structured-additive-IR,google-research/structured-additive-IR