Enable simplify MIR-opt test

tests/mir-opt/simplify_if.rs

@@ -1,10 +1,13 @@
-// skip-filecheck
 // EMIT_MIR_FOR_EACH_PANIC_STRATEGY
 #[inline(never)]
 fn noop() {}
 
 // EMIT_MIR simplify_if.main.SimplifyConstCondition-after-const-prop.diff
 fn main() {
+    // CHECK-LABEL: fn main(
+
+    // CHECK: bb0: {
+    // CHECK-NEXT: return;
     if false {
         noop();
     }