Insert null checks for pointer dereferences when debug assertions are enabled Similar to how the alignment is already checked, this adds a check for null pointer dereferences in debug mode. It is implemented similarly to the alignment check as a `MirPass`. This inserts checks in the same places as the `CheckAlignment` pass and additionally also inserts checks for `Borrows`, so code like ```rust let ptr: *const u32 = std::ptr::null(); let val: &u32 = unsafe { &*ptr }; ``` will have a check inserted on dereference. This is done because null references are UB. The alignment check doesn't cover these places, because in `&(*ptr).field`, the exact requirement is that the final reference must be aligned. This is something to consider further enhancements of the alignment check. For now this is implemented as a separate `MirPass`, to make it easy to disable this check if necessary. This is related to a 2025H1 project goal for better UB checks in debug mode: https://github.com/rust-lang/rust-project-goals/pull/177. r? `@saethlin`