RFR: 8188055: (ref) Add Reference::refersTo predicate [v2]

[Kim Barrett]

> On Oct 8, 2020, at 10:02 AM, Roger Riggs <rriggs at openjdk.java.net> wrote:
> At what point would the @IntrinsicCandidate annotation be added to the
> refersTo0 methods?
> it would be useful documentation even if it is not needed for the mechanism.

I now have a patch to implement the C2 intrinsic, mostly provided by eosterlund.
It is very nearly completely separate from the current PR; the only point of contact
is adding the @IntrinsicCandidate annotations.  Also, the intrinsic needs review
by pretty much a completely different set of people (compiler folks) than the
current PR.

Because of that, I decided to hold off and keep it separate, but I intend to make
a PR for it shortly after the current one is integrated.  Also, the intrinsic needs
review by pretty much a completely different set of people (compiler folks) than
the current PR.

I don’t know what happens if one annotates as an intrinsic candidate with no
intrinsic implementation.  I expect nothing much.  But I originally didn’t want to
do that because it wasn’t initially clear when someone would have time for it,
and I didn’t want to be misleading about the performance in the meantime.