[OpaquePtr] Support changing load type in InstCombine

When the load type is changed to ptr, we need the load pointer type
to also be ptr, because it's not allowed to create a pointer to an
opaque pointer. This is achieved by adjusting the getPointerTo() API
to return an opaque pointer for an opaque pointer base type.

Differential Revision: https://reviews.llvm.org/D104718