Expand comment on disappearing edge.

compiler/rustc_mir_transform/src/jump_threading.rs

@@ -645,7 +645,7 @@ fn apply_once(&mut self, index: usize, body: &mut Body<'_>) {
             debug!(?current, ?succ);
 
             // `succ` must be a successor of `current`. If it is not, this means this TO is not
-            // satisfiable, so we bail out.
+            // satisfiable and a previous TO erased this edge, so we bail out.
             if basic_blocks[current].terminator().successors().find(|s| *s == succ).is_none() {
                 debug!("impossible");
                 return;