Disable two mir opts that are known to be unsound

compiler/rustc_mir_transform/src/separate_const_switch.rs

@@ -46,7 +46,9 @@ pub struct SeparateConstSwitch;
 
 impl<'tcx> MirPass<'tcx> for SeparateConstSwitch {
     fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
-        sess.mir_opt_level() >= 2
+        // This pass participates in some as-of-yet untested unsoundness found
+        // in https://github.com/rust-lang/rust/issues/112460
+        sess.mir_opt_level() >= 2 && sess.opts.unstable_opts.unsound_mir_opts
     }
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {