[Verifier] Allow undef/poison token argument to llvm.experimental.gc.result

As part of the optimization in the unreachable code, we remove
tokens, thereby replacing them with undef/poison in intrinsics.
But the verifier falls on the assertion, within of what it sees
token poison in unreachable code, which in turn is incorrect.

bug: 57871, https://github.com/llvm/llvm-project/issues/57871
Differential Revision: https://reviews.llvm.org/D134427