Optimized builds are broken due to incorrect assert_is_rfp shortcuts

Fails on many platforms, for example arm32:

```
/home/shade/trunks/jdk/src/hotspot/cpu/arm/smallRegisterMap_arm.inline.hpp:36:36: error: expected ';' at end of member declaration
   36 | static void assert_is_rfp(VMReg r) PRODUCT_RETURN
      | ^
```

The reason is that `PRODUCT_RETURN` + `DEBUG_ONLY` are not unity: in "optimized" builds, we have `!PRODUCT` and `!DEBUG`.

```
  static void assert_is_rfp(VMReg r) PRODUCT_RETURN
                                     DEBUG_ONLY({ Unimplemented(); })
```