[CUDA] Fix SYCL NVPTX passes if function operand is null-ref

[rolandschulz]

Fixes #3271

[rolandschulz]

Note: I tested that this fixes the issue. But I don't really understand the code and why it is null in this case. Therefore I don't know whether this is the correct fix and isn't just working around the actual problem.

[bader]

+@steffenlarsen, @Naghasan

[steffenlarsen]

I do not have an explanation as to why the function operand is null-ref, and frankly I am slightly concerned. That said, I am okay with this fix.

[bader]

@rolandschulz, could you add a small regression test exposing the issue, please?
You can try bugpoint tool to reduce the test case from #3271.

[Naghasan]

The null ref is probably coming from the dead argument elimination pass. As it builds a new function and removes the old one, this would turns into a nullref in the MD later on.

Regardless, the checks this patch adds make sense and is safe.