Fix compile-flags directive

Co-authored-by: Ralf Jung <post@ralfj.de>

Author: ecstatic-morse

Diff for: src/test/ui/consts/const-float-bits-conv.rs

@@ -1,4 +1,4 @@
-// -Zmir-opt-level=0
+// compile-flags: -Zmir-opt-level=0
 // run-pass
 
 #![feature(const_panic)]

Diff for: src/test/ui/consts/const-float-classify.rs

@@ -1,4 +1,4 @@
-// -Zmir-opt-level=0
+// compile-flags: -Zmir-opt-level=0
 // run-pass
 
 #![feature(const_panic)]