check mir exists before validation; fix tests

src/test/ui/issues/issue-52023-array-size-pointer-cast.rs

@@ -1,4 +1,3 @@
 fn main() {
     let _ = [0; (&0 as *const i32) as usize]; //~ ERROR casting pointers to integers in constants
-    //~^ ERROR evaluation of constant value failed
 }