Are you suggesting that driver code has an opportunity to run
before core code can implement checks?
If so, then again, this has the fundamental problem that you're expecting the compiler to implement some optimizations - like pushing the dereference out of the buggy driver via LTO - and not others. If the compiler had all optimizations off, it would just generate code to read from x->buffer in the obvious place, which would page-fault at runtime before the check from core code even happened.
If the core code implements the check first, then this optimization won't cause you problems, because this optimization is strictly about checking for a NULL pointer after using it.
For the specific case of computing the address of x->buffer without actually dereferencing it, I do agree that the compiler should not consider that to be an implicit non-null promise. That seems much more likely to be a change you can make without breaking the language spec. See the subthread with 'dbaupp.
(Alternatively, you could work on building tools for driver writers to write in a less error-prone language than C, because drivers being of poor quality is a much broader problem with much broader impacts than just triggering ill-advised optimizations under LTO, and simply optimizing drivers less efficiently isn't going to solve the problem that the driver shouldn't be dereferencing x->buffer without its own NULL check in the first place. If you check my recent GitHub activity you might notice a project to do exactly this in Linux....)