[mlir][LLVM] Support opaque pointers in `llvm.mlir.addressof`

The verifier of llvm.mlir.addressof did not properly account for opaque pointers, that is, the pointer type not having an element type equal to the type of the referenced global or function. This patch fixes that by skipping the test for the element type if the pointer is opaque.

Differential Revision: https://reviews.llvm.org/D124333