RFR: Simpler obj equals barrier for C2

[Roland Westrelin]

> There's 3 affected intrinsics:
> - isInterrupted: this one did not actually change
> - subtype-check: this seems likely to trigger, and in-fact I have seen
> it trigger because I had a bug in my barrier
> - aes-crypt: this one will likely not trigger by the test-suite, but by
> specjvm. I'm running it now.

java -XX:-TieredCompilation -XX:+CompileTheWorld -XX:+UseShenandoahGC -version

will stress test everything.

> WBs work in all other places, and produces the memphis that it needs,
> afaik. I don't really understand why it wouldn't here.

In other places, parsing builds the control flow and adds the phis. With
intrinsics, we add explicit control flow and so we must add phis
ourselves because nothing else does it for us.

Roland.