RFR(sh/jdk11): Shenandoah: Isolate shared-code changes

[Roland Westrelin]

> Hmm, but it does exist in jdk11u and the intention here was to make clear
> that we don't possibly change existing behaviour by introducing Shenandoah
> to upstream jdk11u. E.g. compile to the exact same code when building with
> -shenandoahgc.

I understand but it feels silly that the easiest forward is to make a
useless change. Anyway, I don't object to it.

Roland.