Ignore now-broken mir-opt test

src/test/mir-opt/simplify_try_if_let.rs

@@ -1,4 +1,7 @@
 // compile-flags: -Zmir-opt-level=1 -Zunsound-mir-opts
+// ignore-test
+// FIXME: the pass is unsound and causes ICEs in the MIR validator
+
 // EMIT_MIR simplify_try_if_let.{impl#0}-append.SimplifyArmIdentity.diff
 
 use std::ptr::NonNull;
@@ -19,7 +22,7 @@ impl LinkedList {
 
     pub fn append(&mut self, other: &mut Self) {
         match self.tail {
-            None => { },
+            None => {}
             Some(mut tail) => {
                 // `as_mut` is okay here because we have exclusive access to the entirety
                 // of both lists.