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 is related to a 2025H1 project goal for better UB checks in debug
mode: https://github.com/rust-lang/rust-project-goals/pull/177.

compiler/rustc_mir_transform/src/check_pointers.rs

@@ -17,6 +17,7 @@ pub(crate) struct PointerCheck<'tcx> {
 /// [NonMutatingUseContext::SharedBorrow].
 #[derive(Copy, Clone)]
 pub(crate) enum BorrowCheckMode {
+    IncludeBorrows,
     ExcludeBorrows,
 }
 
@@ -168,7 +169,7 @@ impl<'a, 'tcx> PointerFinder<'a, 'tcx> {
             ) => true,
             PlaceContext::MutatingUse(MutatingUseContext::Borrow)
             | PlaceContext::NonMutatingUse(NonMutatingUseContext::SharedBorrow) => {
-                !matches!(self.borrow_check_mode, BorrowCheckMode::ExcludeBorrows)
+                matches!(self.borrow_check_mode, BorrowCheckMode::IncludeBorrows)
             }
             _ => false,
         }