[OpaquePtr] Support opaque pointers in intrinsic type check

This adds support for opaque pointers in intrinsic type checks
of IIT kind Pointer and PtrToElt.

This is less straight-forward than it might initially seem, because
we should only accept opaque pointers here in --force-opaque-pointers
mode. Otherwise, there would be more than one valid type signature
for a given intrinsic name.

Differential Revision: https://reviews.llvm.org/D105155