Fix test.

Author: reitermarkus

tests/mir-opt/instsimplify/combine_transmutes.rs

@@ -1,9 +1,9 @@
 // unit-test: InstSimplify
 // compile-flags: -C panic=abort
-
 #![crate_type = "lib"]
 #![feature(core_intrinsics)]
 #![feature(custom_mir)]
+#![feature(generic_nonzero)]
 
 use std::intrinsics::mir::*;
 use std::mem::{MaybeUninit, ManuallyDrop, transmute};