[RFC] Redefine `convergent` in terms of dynamic instances

Hi all,

this is a follow-up to the thread about adding thread group semantics
to LangRef from last December, see here:

Please see the concrete proposal at ⚙ D68994 [RFC] Redefine `convergent` in terms of dynamic instances
for a direction to take to address the `convergent` issues. It
contains both a high-level abstract of the what & why, as well as the
proposed LangRef changes and new intrinsics.

A full solution through an entire compiler stack will require proper
frontend and backend support of the new intrinsics, but this RFC patch
should give you an idea of how things will work. and hopefully can
serve as a basis for making an informed decision whether this approach
can go upstream.

Please let me know what you think. I'd be happy to discuss the
proposal by email and next week at the LLVM Dev Meeting.