Prevent a test from seeing forbidden numbers in the rustc version

The final CHECK-NOT directive in this test was able to see past the end of the
enclosing function, and find the substring 753 or 754 in the git hash in the
rustc version number, causing false failures in CI.

Adding an explicit check for `ret` prevents the CHECK-NOT directive from seeing
past the end of the function.

Author: Zalathar

Diff for: tests/codegen/issues/issue-122600-ptr-discriminant-update.rs

@@ -37,6 +37,8 @@ pub unsafe fn update(s: *mut State) {
     // CHECK-NOT: store
     // CHECK-NOT: memcpy
     // CHECK-NOT: 75{{3|4}}
+
+    // CHECK: ret
     let State::A(v) = s.read() else { std::hint::unreachable_unchecked() };
     s.write(State::B(v));
 }