Set mir-opt-level to 3 to prevent regressions

Author: JohnTitor

src/test/mir-opt/inline/inline-into-box-place.rs

@@ -1,5 +1,6 @@
 // ignore-tidy-linelength
 // ignore-wasm32-bare compiled with panic=abort by default
+// compile-flags: -Z mir-opt-level=3
 #![feature(box_syntax)]
 
 fn main() {