Distinguish between library and lang UB in assert_unsafe_precondition

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -685,7 +685,8 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                         let val = layout.offset_of_subfield(bx.cx(), fields.iter()).bytes();
                         bx.cx().const_usize(val)
                     }
-                    mir::NullOp::DebugAssertions => {
+                    mir::NullOp::UbCheck(_) => {
+                        // In codegen, we want to check for language UB and library UB
                         let val = bx.tcx().sess.opts.debug_assertions;
                         bx.cx().const_bool(val)
                     }