[opaque pointer type] Access the pointee of the result type from the GEP...

[opaque pointer type] Access the pointee of the result type from the GEP rather than pulling it out of the pointer result type

The implementation of this GEP::getResultElementType will be refactored
to either rely on a member variable, or recompute the value from the
indicies (any preferences?).

llvm-svn: 235236