Add test case which demonstrates current miscomp.

Author: jumbatm

src/test/ui/mir/issue67529.rs

@@ -0,0 +1,14 @@
+// Tests for miscompilation due to const propagation, as described in #67529. This used to result
+// in an assertion error, because d.a was replaced with a new allocation that was never
+// initialized.
+//
+// run-pass
+// compile-flags: -Zmir-opt-level=2
+struct Baz<T: ?Sized> {
+    a: T,
+}
+
+fn main() {
+    let d: Baz<[i32; 4]> = Baz { a: [1, 2, 3, 4] };
+    assert_eq!([1, 2, 3, 4], d.a);
+}