Eliminate UbCheck for non-standard libraries

compiler/rustc_middle/src/mir/syntax.rs

@@ -1361,8 +1361,8 @@ pub enum NullOp<'tcx> {
     AlignOf,
     /// Returns the offset of a field
     OffsetOf(&'tcx List<(VariantIdx, FieldIdx)>),
-    /// Returns whether we want to check for UB.
-    /// This returns the value of `cfg!(debug_assertions)` at monomorphization time.
+    /// Returns whether we should perform some UB-checking at runtime.
+    /// See the `ub_checks` intrinsic docs for details.
     UbChecks,
 }