[llvm] [llvm] Migrate away from PointerUnion::dyn_cast (NFC) (PR #123692)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

> @nikic Thank you for pointing out the opportunities to call `dyn_cast<T>` and `cast<T>`. Let me create a separate PR for these non-mechanical migrations.

I've created https://github.com/llvm/llvm-project/pull/123716 for the non-mechanical migrations.  Thanks!



https://github.com/llvm/llvm-project/pull/123692