[llvm] r265514 - [IRVerifier] Avoid crashing on an invalid compile unit.

[Rafael Espíndola via llvm-commits]

> +    if (isa<DICompileUnit>(CU))
> +      if (auto Ts = cast<DICompileUnit>(CU)->getRetainedTypes())

Use dyn_cast instead of isa + cast.

Cheers,
Rafael