Fix collector opt comment

Author: Sa4dUs

compiler/rustc_monomorphize/src/collector/autodiff.rs

@@ -5,8 +5,8 @@ use crate::collector::{MonoItems, create_fn_mono_item};
 
 // Here, we force both primal and diff function to be collected in
 // mono so this does not interfere in `autodiff` intrinsics
-// codegen process. If they are unused, they will be removed later and
-// won't be present at LLVM-IR.
+// codegen process. If they are unused, LLVM will remove them when 
+// compiling with O3.
 pub(crate) fn collect_autodiff_fn<'tcx>(
     tcx: TyCtxt<'tcx>,
     instance: ty::Instance<'tcx>,