Fix missing return in optimization_applies()

Author: wesleywiser

src/librustc_mir/transform/simplify_try.rs

@@ -302,6 +302,7 @@ fn optimization_applies<'tcx>(
     // Verify the assigment chain consists of the form b = a; c = b; d = c; etc...
     if opt_info.field_tmp_assignments.is_empty() {
         trace!("NO: no assignments found");
+        return false;
     }
     let mut last_assigned_to = opt_info.field_tmp_assignments[0].1;
     let source_local = last_assigned_to;