Leave FIXME for wasm layout difference.

There is a distinction between running this on wasm and i686, even though they should be
identical. This technically is not _incorrect_, it's just an unexpected difference, which is
worth investigating, but not for correctness.

compiler/rustc_mir_transform/src/lib.rs

@@ -1,8 +1,8 @@
 #![allow(rustc::potential_query_instability)]
 #![feature(box_patterns)]
 #![feature(drain_filter)]
+#![feature(box_syntax)]
 #![feature(let_chains)]
-#![feature(let_else)]
 #![feature(map_try_insert)]
 #![feature(min_specialization)]
 #![feature(never_type)]