[clang] [OpenACC][CIR] Start work to lower 'loop' (PR #137972)

[Erich Keane via cfe-commits]

erichkeane wrote:

> > In the future, we'll have to teach the OpenACC dialect how to derive the upperbound, lowerbound, and step from the cir.for loop
> 
> Yea, even better if the `cir.for` representation can itself be improved to expose that information (either by transforming it into a more refined loop op or by redesigning the existing one).

Agreed! I  suspect this will be a bit of a 'meet in the middle' for some of it, but it is hopefully clear that the the CIR and ACC dialect folks can/will work together to modify the loops to work together better.

https://github.com/llvm/llvm-project/pull/137972