Unify the const folding errors

before they differed depending on whether optimizations were on or not

src/test/ui/const-eval/conditional_array_execution.rs

@@ -10,7 +10,8 @@
 
 const X: u32 = 5;
 const Y: u32 = 6;
-const FOO: u32 = [X - Y, Y - X][(X < Y) as usize]; //~ E0080
+const FOO: u32 = [X - Y, Y - X][(X < Y) as usize];
+//~^ WARN attempt to subtract with overflow
 
 fn main() {
     println!("{}", FOO); //~ E0080