Simplify code flow.

Author: cjgillot

compiler/rustc_mir_transform/src/lib.rs

@@ -1,6 +1,7 @@
 #![allow(rustc::potential_query_instability)]
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
+#![feature(assert_matches)]
 #![feature(box_patterns)]
 #![feature(cow_is_borrowed)]
 #![feature(decl_macro)]