[opaque pointer type] Allow gep_type_iterator to work with the pointee type...

[opaque pointer type] Allow gep_type_iterator to work with the pointee type from the GEP instruction

The raw non-instruction/constant form of this is still relying on being
able to access the pointee type from a pointer type - those will be
cleaned up later. For now, just focus on the cases where the pointee
type is easily accessible.

llvm-svn: 237958