some UB gets masked by optimizations

Author: RalfJung

tests/compile-fail/dangling_pointers/dyn_size.rs

@@ -1,5 +1,5 @@
-// should find the bug even without these
-// compile-flags: -Zmiri-disable-validation -Zmiri-disable-stacked-borrows
+// should find the bug even without these, but gets masked by optimizations
+// compile-flags: -Zmiri-disable-validation -Zmiri-disable-stacked-borrows -Zmir-opt-level=0
 
 struct SliceWithHead(u8, [u8]);
 

tests/compile-fail/unaligned_pointers/dyn_alignment.rs

@@ -1,5 +1,5 @@
-// should find the bug even without these
-// compile-flags: -Zmiri-disable-validation -Zmiri-disable-stacked-borrows
+// should find the bug even without these, but gets masked by optimizations
+// compile-flags: -Zmiri-disable-validation -Zmiri-disable-stacked-borrows -Zmir-opt-level=0
 
 #[repr(align(256))]
 #[derive(Debug)]

tests/compile-fail/validity/nonzero.rs

@@ -1,4 +1,5 @@
-// compile-flags: -Zmir-opt-level=1
+// gets masked by optimizations
+// compile-flags: -Zmir-opt-level=0
 #![feature(rustc_attrs)]
 #![allow(unused_attributes)]