C's notion of null _is_ the _only_ relevant

Determining that code is dead is easy (and I heartily approve it) if the actual values can be computed at compile time. For the particular case you mention (all callers pass pointers which are known not to be null), you would probably need whole-program optimisation to determine it.

However, knowing that the program has already attempted to dereference a pointer is not quite the same as statically determining that the pointer is definitely non-NULL.

I submit that removing such a test when some possible sources of the value are not visible to the compiler is an excessive optimisation and warrants a warning.

People write defensive code for a reason.