[llvm] [llvm] Use dyn_cast_if_present instead of dyn_cast_or_null (NFC) (PR #75733)

[Nikita Popov via llvm-commits]

https://github.com/nikic requested changes to this pull request.

I don't agree that we should use the `if_present` style APIs in places that work specifically on pointers, where `nonnull` is a lot clearer than `present`.

https://github.com/llvm/llvm-project/pull/75733