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_const_eval/src/const_eval/machine.rs

@@ -508,6 +508,7 @@ impl<'tcx> interpret::Machine<'tcx> for CompileTimeMachine<'tcx> {
                     found: eval_to_int(found)?,
                 }
             }
+            NullPointerDereference => NullPointerDereference,
         };
         Err(ConstEvalErrKind::AssertFailure(err)).into()
     }