Add FileCheck to partial_init.rs

tests/mir-opt/copy-prop/partial_init.rs

@@ -1,4 +1,3 @@
-// skip-filecheck
 //@ test-mir-pass: CopyProp
 // Verify that we do not ICE on partial initializations.
 
@@ -9,6 +8,9 @@
 // EMIT_MIR partial_init.main.CopyProp.diff
 #[custom_mir(dialect = "runtime", phase = "post-cleanup")]
 pub fn main() {
+    // CHECK-LABEL: fn main(
+    // CHECK: let mut [[x:_.*]]: (isize,);
+    // CHECK: ([[x]].0: isize) = const 1_isize;
     mir! (
         let x: (isize, );
         {