account for mir-opts masking more errors

tests/compile-fail/storage_dead_dangling.rs

@@ -1,5 +1,5 @@
-// This should fail even without validation
-// compile-flags: -Zmiri-disable-validation
+// This should fail even without validation, but some MIR opts mask the error
+// compile-flags: -Zmiri-disable-validation -Zmir-opt-level=0
 
 static mut LEAK: usize = 0;
 

tests/compile-fail/validity/transmute_through_ptr.rs

@@ -1,4 +1,5 @@
 #[repr(u32)]
+#[derive(Debug)]
 enum Bool { True }
 
 fn evil(x: &mut Bool) {
@@ -9,6 +10,7 @@ fn evil(x: &mut Bool) {
 fn main() {
     let mut x = Bool::True;
     evil(&mut x);
-    let _y = x; // reading this ought to be enough to trigger validation
+    let y = x; // reading this ought to be enough to trigger validation
     //~^ ERROR encountered 0x0000002c at .<enum-tag>, but expected a valid enum tag
+    println!("{:?}", y); // make sure it is used (and not optimized away)
 }