Fix UB in the test case

Author: saethlin

tests/mir-opt/copy-prop/custom_move_arg.f.CopyProp.diff

@@ -7,8 +7,9 @@
       let mut _3: NotCopy;                 // in scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
 
       bb0: {
--         _2 = move _1;                    // scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
-          _0 = opaque::<NotCopy>(move _1) -> bb1; // scope 0 at $DIR/custom_move_arg.rs:+3:9: +3:41
+-         _2 = _1;                         // scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
+-         _0 = opaque::<NotCopy>(move _1) -> bb1; // scope 0 at $DIR/custom_move_arg.rs:+3:9: +3:41
++         _0 = opaque::<NotCopy>(_1) -> bb1; // scope 0 at $DIR/custom_move_arg.rs:+3:9: +3:41
                                            // mir::Constant
                                            // + span: $DIR/custom_move_arg.rs:15:24: 15:30
                                            // + literal: Const { ty: fn(NotCopy) {opaque::<NotCopy>}, val: Value(<ZST>) }

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

@@ -11,7 +11,7 @@ struct NotCopy(bool);
 #[custom_mir(dialect = "analysis", phase = "post-cleanup")]
 fn f(_1: NotCopy) {
     mir!({
-        let _2 = Move(_1);
+        let _2 = _1;
         Call(RET, bb1, opaque(Move(_1)))
     }
     bb1 = {