[OpaquePtr] Return opaque pointer from opaque pointer GEP

For a GEP on an opaque pointer, also return an opaque pointer (or
vector of opaque pointer) result.

This requires explicitly enumerating the GEP source element type,
because it is now no longer implicitly enumerated as part of either
the source or result pointer types.

Differential Revision: https://reviews.llvm.org/D104652