[opaque pointer type] Avoid using PointerType::getElementType when parsing IR

A few calls are left in for error checking - but I'm commenting those
out & trying to build some IR tests (aiming for Argument Promotion to
start with). When I get any of these tests passing I may add flag to
disable the checking so I can add tests that pass with the assertion in
place.

llvm-svn: 234206