add some comments explaining why MIR opts are marked as unsound

compiler/rustc_mir_transform/src/large_enums.rs

@@ -30,6 +30,9 @@ pub struct EnumSizeOpt {
 
 impl<'tcx> MirPass<'tcx> for EnumSizeOpt {
     fn is_enabled(&self, sess: &Session) -> bool {
+        // There are some differences in behavior on wasm and ARM that are not properly
+        // understood, so we conservatively treat this optimization as unsound:
+        // https://github.com/rust-lang/rust/pull/85158#issuecomment-1101836457
         sess.opts.unstable_opts.unsound_mir_opts || sess.mir_opt_level() >= 3
     }
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {